History Main / EnemyWithin

* ''Fanfic/StairwayToEquestria'': Similarly to Luna, Midnight Blink has in inner Nightmare of his own that he has to deal with constantly throughout part two and three of the story.

Either way, the enemy is behind the hero's eyes, and its time is coming when [[SplitPersonalityTakeover it can take over.]] Until then, it'll do all it can [[GollumMadeMeDoIt to control him]] and get him to give in to its HorrorHunger. The thing to stress most is that the Enemy Within ''is'' the hero. He or she cannot simply exorcise it out. Often the Enemy Within is [[SuperpoweredEvilSide the cause of the powers that the hero has]] that allows them to do what they do. WithGreatPowerComesGreatInsanity, remember?

* ''Manga/JoJosBizarreAdventure'':
** ''[[Manga/JoJosBizarreAdventureDiamondIsUnbreakable Diamond is Unbreakable]]'': Masazo Kinoto wasn't strong-willed enough to control his Stand, Cheap Trick, so it manifested as an externalization of his own fears and went along with Yoshihiro Kira's plan to kill Rohan, knowing that it would kill its own user.
** ''[[Manga/JoJosBizarreAdventureGoldenWind Golden Wind]]'': [[spoiler:Silver Requiem]] is capable of invoking this when any direct attack will result in a person's own Stand turning against them. [[spoiler:Diavolo pulls this off when he hid his soul within Trish in an attempt to ambush the team]].

* One of the HunterOfMonsters duo from ''Manga/VenusVersusVirus'' is a young girl who is an "Anti-Virus," a biological evolution against TheHeartless monsters that plague mankind. However, this has resulted in a SplitPersonality that merely kills TheHeartless first, and then turns on her partner. The FlashForward at the beginning of the first episode implies that her two halves will become whole... but as a FaceHeelTurn. [[spoiler:In the manga she eventually learns how to control her virus side and use it to her advantage.]]
* ''Manga/{{Berserk}}'': The Beast of Darkness is the personification of all of Guts's hatred and bloodlust which was born from the horrors he suffered during the Eclipse and from the two years he spent WalkingTheEarth as the Black Swordsman, seeking vengeance against Griffith and every demon he could find. After Guts decided to stop focusing on revenge in order to focus on Casca, his only love and the only other survivor of the Eclipse, it begins to act against him, and seeks to have Guts [[MoralityChain kill her]] so that Guts can get back to his vendetta against Griffith.
* [[spoiler:Reito]] from ''Anime/MyHime'' has one, and it's [[spoiler:the BigBad, the alien Obsidian Lord who is possessing him and using him to manipulate Mai]].
* Ryo Takatsuki from ''Manga/ProjectARMS'' has an Enemy Within. Specifically, within his right arm. [[SuperpoweredEvilSide Tick him off enough, and he turns into the Jabberwock, a massive demonic humanoid capable of mass destruction.]] [[Literature/AliceInWonderland Beware the Jabberwock my son!]]
** There's also a variant with [[spoiler:Alice, who split into Dark and White Alice, upon fusing with an alien lifeform. While Dark Alice isn't ''within'' White Alice, she does represent her dark desires and hatred for humanity, and White Alice is shown to be horrified by her actions, but almost completely helpless to stop them.]]
* Manga/InuYasha has this in the form of his demon blood. Being a half demon, half human, his demon blood is considered too powerful for him to control. When it takes control, it turns him into a mindless animal that attacks purely on instinct, unable to tell friend from foe. Inuyasha's father gave him a safeguard in the form of his EmpathicWeapon Tessaiga to keep it in check. As long as the sword remains in Inuyasha's possession, his demon side is suppressed, though he remains stronger than the average human for it.
* Senri from ''Manga/PlusAnima'' has an [[spoiler:eyepatch in order to keep his +Anima in check, without it he goes berserk]].
* In ''Manga/DGrayMan'', it's revealed that [[spoiler:the 14th Noah wants to kill [[BigBad The Millennium Earl]] so ''he'' can become the next Millennium Earl. [[AllLovingHero His host]], on the other hand, would rather not.]]
* In ''Anime/DragonBallZ'', [[OurGeniesAreDifferent Majin Buu]] was introduced as a huge, pink, disturbingly cheerful jinn that liked to [[BodyHorror turn people into candy]] and [[IAmAHumanitarian eat them]]. However, an encounter with [[AdoptTheDog an injured dog]] and [[JerkWithAHeartOfGold Hercule/Mr. Satan]] caused a HeelFaceTurn. Crisis averted, right? Well, up until the dog gets shot, at which point he becomes so furious that [[EnemyWithout all of his inner evil ''comes to life'']]. It then ''eats him, becoming even more powerful than the original as a result.'' Becomes [[spoiler:inverted when the good part of Buu struggles inside of himself in a desperate attempt to save Hercule/Mr. Satan, becoming the Ally Within you could say.]]

* The eponymous character of ''Manga/SoulEater'' deals with this, especially as it pertains to keeping Maka safe. As a result of his and Maka's first battle with Crona, he develops a small ogre-demon living inside his soul. It starts to take over in the final battle with Asura ([[GeckoEnding in the anime]]), when Maka has to [[spoiler:go into Soul's soul and save the real Soul from the ogre-demon, who has disguised himself to look exactly like the guy]], in order to save her partner from the Madness with the PowerOfLove.
* Remember that nice young man [[MysteriousProtector Jack Vessalius]] in ''Manga/PandoraHearts'' who helps [[TheHero Oz]] find his girlfriend, saves their butts a lot, and protects him? [[spoiler:He's not a good person. He's FauxAffablyEvil.]]

* ''Anime/VariableGeo'' has a different take on the trope. Back when [[LadyOfWar Reimi]] was a child, she was subjected to inhumane experiments, during which a bio-tech "seed" was surgically planted within her body, under [[EvilMatriarch her mother's]] orders. This would become [[spoiler:[[BigBad Miranda]]]]'s trump card, years later, when she [[spoiler:activates it, causing it to sprout into a vine covered in white roses, which saps Reimi's strength]], during her match with [[{{deuteragonist}} Satomi]]. Thereby, cheating Reimi out of her victory and eliminating her from the competition.
* In ''Manga/{{Claymore}}'', all of the Claymores deal with this. While they can enhance their strength and speed by getting in touch with their Yoma side, they also place their humanity at risk. Most can control their dark side, but others submit to it and turn on their friends and the human race.
* Ryner Lute from ''Literature/TheLegendOfTheLegendaryHeroes''. The three inhabitants of his mind include a being that results in a SuperpoweredEvilSide, a generally benevolent being, and [[spoiler:his mother]], the last of whom exists to try and counteract the first one.

* ''Manga/KotouraSan'' plays this one very similarly to ''Anime/MyHime'''s example above. [[spoiler:Detective Tsukino]] has one and it's [[spoiler:the actual criminal behind the beatings/rapings of high school girls]] throughout Episodes 9 though 11. [[{{Telepathy}} Haruka]] defeats this one with IKnowYoureInThereSomewhereFight in the most literal way possible.
* In ''Manga/RGVeda'', [[spoiler:Ashura has one that represents [[OmnicidalManiac the true nature]] of [[PersonOfMassDestruction his kin]], and it's revealed that the whole plot is result of an elaborate plan to allow him to be born without the fear of his true persona awakening in order to destroy everything.]]
* ''Franchise/KagerouProject'': The Wide-Open Eyes Snake, the BigBad of the series, is said to have first appeared to Azami in a dream, [[spoiler:offering to teach her how to create a whole new world so she could live with her family forever]]. Like all of the other snakes, he is implied to have been born from Azami's emotions - in this case, [[spoiler:her desperation and willingness to do anything to live on with her husband and daughter]]. The Snake is also the only sentient Snake [[spoiler:aside from Shintaro's Recording Eyes]], and capable of taking complete control of its master when they are asleep.
* ''Manga/OPartsHunter'': Jiro carries the Satan recipe within him who naturally isn't at all nice. While he does offer Jiro some power to keep the edge in fights, especially against Kaballah recipes. Satan ultimately wishes to take over Jiro body and take over the world. [[spoiler: Interestingly we find out ''Jiro'' was a personality that had broken off from Satan and was given the driving seat in their body, so Satan is technically trying to get back what's his.]]
* In ''Manga/TheLegendOfZeldaTwilightPrincess2016'', contrasting the original game, [[spoiler:Dark Link is a manifestation of Link's InferioritySuperiorityComplex and his repeatedly lamenting over how unfit he is to be a hero. Once Link gets the Master Sword, he becomes very overconfident and violent. After Link loses a fight to him, and then to Zant, Dark Link rubs everything into his face]].

** In the ''ComicBook/{{Superman}}'' story ''ComicBook/WarWorld'', Superman's final test is confronting his dark side, an uncontrolled Superman that is the embodiment of his capacity for violence.

* Malus Darkblade and T'zarkan in the ''Literature/MalusDarkblade'' series. Malus is already a very evil person and as such doesn't exactly need goading, but T'zarkan tempts him with power at the cost of more of his soul.

** ''ComicBook/XMen'':
*** ComicBook/{{Wolverine}} is an interesting example, as his own Enemy Within isn't really some demonic force within him but rather part of his mutant biology. His mutation gives him a [[TheBerserker 'berserker rage']] that he constantly struggles to keep under control so he can retain his humanity; generally, the best he can do is reconcile parts of his humanity with his berserker rage to give himself an anti-authoritarian personality. On the other hand, his ArchEnemy [[Characters/MarvelComicsSabretooth Sabretooth]], who has a similar mutation, has [[FullyEmbracedFiend completely embraced]] his rage.
*** Wolverine's OppositeSexClone surrogate daughter Laura Kinney, a.k.a. ComicBook/{{X 23}}, has a similar problem that was implanted in her by the Facility that created her. A special "Trigger Scent" awakens the berserker within her. In this state, she won't stop until whatever or whoever was exposed to the Trigger Scent is dead. The worst part is that unlike Logan, she cannot resist her Enemy Within. She's killed a lot of innocent people because of the Trigger Scent.

* ''ComicBook/TheTransformersMarvel'': One timeline had Rodimus Prime trapping Unicron inside the Matrix... which was located in Rodimus' chest. It wasn't pretty when Unicron managed to break free and possess the young Prime's body.

* [[Manga/LuckyStar Kagami]] in ''Fanfic/StarsAbove'' [[spoiler:has an OmnicidalManiac EldritchAbomination inside her body, which will burst free and destroy TheMultiverse if she [[DespairEventHorizon gives into despair]]. It whispers to her in times of great stress, trying to convince her to let it out.]]
* [[spoiler:Loneliness]], the first BigBad of the ''Fanfic/PonyPOVSeries'', is this to [[spoiler:Trixie]], keeping her practically comatose for ''weeks'' even after Discord's defeat until [[spoiler:Twilight [[JourneyToTheCenterOfTheMind enters Trixie's mind]] and helps Trixie defeat her]]. What's [[NothingIsScarier truly terrifying]] about her is that it's never made clear exactly ''what'' she is or where she came from.
** Fluttercruel was originally an ineffective one of these, being more a mild annoyance to Fluttershy than anything else, and even when she finally manages to take control of Fluttershy's body, all she does is party, rather than anything evil... though her actions ''do'' accidentally lead to [[spoiler:Fluttershy becoming [[SuperpoweredEvilSide Nightmare Whisper]]]]. But that just makes her have a HeelFaceTurn, and she and Fluttershy now [[SharingABody share a body]] and get along very well (they both consider Cruel to be Shy's "daughter").
** In the Shining Armor Arc [[spoiler:it's revealed Minuette is actually [[Series/DoctorWho the Master]] using the fob watch and he serves as her storyline's BigBad trying to tempt her into [[SealedInsideAPersonShapedCan opening it and becoming him again]], despite her strong desire not to. In the end, she manages to rid herself of him once and for all by feeding him to [[ClockRoaches the Blank Wolf]], thus [[RetGone erasing him from existence]].]]
* ''Fanfic/StairwayToEquestria'': Similarly to Luna, Midnight Blink has in inner Nightmare of his own that he has to deal with constantly throughout part two and three of the story.
* Kali [[spoiler:with Kira]] in the Death Note fanfic ''Zenith, Darkness, Reverie''. If Kali has a callous disregard for human existence, [[spoiler:[[SuperpoweredEvilSide Kira]] is this]], with a penchant for murder and determination to assert control of Kali's body to do so. [[spoiler:Kira also considers Kali's emotions to be a "vulnerability", despite her relatively obvious lack of sentimentality. This is Kira's primary justification for killing the protagonist. Unfortunately, Kali is not as expendable as Kira would prefer.]]
* ''Fanfic/InnerDemons'': The plot is driven by Twilight's dark side awakening and eventually consuming her mind, making her become the foretold [[BigBad Queen of Darkness]]. [[spoiler:Becomes an EnemyWithout during the FinalBattle, when the Elements of Harmony force the Darkness out of Twilight's body, causing it to take on a form of its own.]]

* ''Fanfic/MegaManReawakened'' has a personification of Robert's hatred, seen as an EvilTwin.
* ''Fanfic/TheLionKingAdventures'': Twice, Simba is faced with mental battles with the King of Dreams, the embodiment of his dark side. Ultimately, he's defeated for good when [[spoiler:Simba decides to embrace and control his dark side, denying the King any power and destroying him]].
* In the spinoff AU for ''Roleplay/TheStrexFamily'', ''Compliance'', [[spoiler:Carlos]] has an inner voice goading him to kill his captors in a variety of painful ways. [[spoiler:Compliance ends with the WhamMoment that the voice took over during the climactic scene, and for the entirety of the sequel, ''Procedure'', he's [[JumpingOffTheSlipperySlope jumped off the slippery slope]] and let the Enemy Within control him.]]
* ''[[http://archiveofourown.org/works/3297710/chapters/8334748 In your arms]]'', [[spoiler:Molly]] succumbs to the [[Literature/TheDresdenFiles Winter Mantle]] she bears, which is signified by her dialogue becoming more antiquated and threatening. [[spoiler:Molly]] is functionally unconscious while this happens, and horrified when she 'wakes up' next to her bloodied, [[AttemptedRape naked]] LoveInterest.
* In ''Fanfic/{{Flinch}}'' trilogy of ''Super Smash Bros'' fics, Sheik is Zelda's enemy within. Zelda is a terrible fighter in part because she is too kind, while Sheik has no such issues. She's a sadistic BloodKnight who likes to cause her opponents maximum pain before they are forced out of the fight. In the second story Zelda locks Sheik away but she ends up coming back. [[spoiler:In the final story Zelda has come to terms with her aggression and gives Samus a NoHoldsBarredBeatdown herself]].
* ''Fanfic/ThePowerOfTheEquinox'': After [[TheHeartless the Entity]] takes root in Twilight Sparkle's body, transforming her into [[SuperPoweredEvilSide Dimmed Star]] in the process, it takes residence in her subconscious. While it intends to possess her completely (and manages a few times to drive her into UnstoppableRage by influencing her thoughts), it's held at bay by the light that represents her positive emotions.
* ''Fanfic/LostTogether'': The amnesiac Ranko has a nightmare in which her former identity -- male Ranma -- assaults her because she's "ruining everything" and is terrified by the prospect of meeting him again. Of course, from Ranma's viewpoint, ''Ranko'' is the enemy since she has replaced him as the dominant personality and her making a life for herself as a woman could mean he would never wake up again.

* ''Film/FightClub'': [[spoiler:Durden wouldn't phrase his relationship with the narrator this way.]]
* ''Film/BlackSwan'' shows a truly horrifying example of this, culminating in both sides of the lead trying to [[MindScrew KILL each other.]]
* ''Film/{{Prevenge}}'': It's left ambiguous, but heavily implied, that Ruth was only inventing the voice of her fetus talking to her as an outlet for her own rage and grief.
* In ''Film/{{Upgrade}}'', [[spoiler:STEM]] turns out to have been this all along, having orchestrated almost everything that transpired in an effort to get an untainted, healthy human body. [[spoiler:[[SplitPersonalityTakeover It succeeds]]]].
* In ''Film/TheDeadCenter'', John Doe is stuck in an emergency psych ward. He spends most of the time either a wheezing, physical wreck or in a catatonic state, making it harder for his psychiatrist to figure out what's going on. Turns out, these are side effects of DemonicPossession. [[spoiler:[[SplitPersonalityTakeover The demon wins]].]]

* In the ''Literature/HIVESeries'', [[spoiler:Overlord]] takes over the body of [[spoiler:Number One]] before the start of the series with the intention of [[BodySurf body surfing]] to [[spoiler:Otto, when he gets old enough. Otto]] foils his plans by apparently killing [[spoiler:Overlord,]] who shows up again in the very next book as a [[spoiler:seed in Otto's mind who pretends to help him,]] when in reality he is only gaining his trust so that he can eventually take over the body of [[spoiler:his son.]]
* White Court Vampires in ''Literature/TheDresdenFiles''. It's called their Demon and their Hunger, and only Thomas even tries to fight it. It exerts control especially in the presence of the relevant emotional "food", though if the vampire tries to starve it out they go mad with Hunger and mindlessly try to Feed. It can be killed if the host experiences the true emotion related to their feeding - love vs lust, etc, but only if it's an adolescent. Thomas has already killed at least one person, so his Hunger can't be killed without also killing him.

* Rand al'Thor and Lews Therin Telamon in Robert Jordan's ''Literature/TheWheelOfTime'' series. [[spoiler:Rand had access to Lews Therin's memories because he is his reincarnation, but the actual Lews Therin "voice" was just a symptom of his madness and identity issues. He sorted that out with a SplitPersonalityMerge as of the]] end of the 12th book.
* Dandra in ''Literature/TheDragonBelow'' trilogy has a rather complicated version in the form of the imprisoned personality of her creator, Tetkashtai.
* Nearra in the first six books of ''Dragonlance: The New Adventures'' has one.
* In Creator/JohnDicksonCarr's ''The Devil in Velvet'', the protagonist, Nicholas, makes a deal with the Devil to return in time to 18th century England to stop a murder most foul by, in essence, leaping INTO and taking over control of the body of his ancient ancestor. But the old boy is still there, trying to get out of the box, and whenever Nicholas is extremely stressed or emotional, Old Nick jumps out, seizes control of his body, and does dastardly things until our hero can regain control.
* ''Literature/{{Discworld}}'':
** Sam Vimes contains an UnstoppableRage ("the Beast") which he first kept in check with alcohol and then with [[HeroicWillpower rigid self-control]]. Sometimes he and the Beast have a common goal, though, such as when his family is threatened.
** Granny Weatherwax (the Discworld's greatest witch) is also hinted to have a much darker side which she keeps in check with her insistence on doing what is Right. After she's bitten by a vampire in ''Literature/CarpeJugulum'', she faces down this dark side in a sort of BattleInTheCenterOfTheMind.
* In ''[[TabletopGame/ForgottenRealms Return of the Archwizards]]'' Galaeron Nihmedu overused the [[ToxicPhlebotinum shadow magic]] despite his mentor's warning. This puts him into the "shadow crisis" -- that is, now his "shadow self" got a foothold in him and it sucks to be Galaeron very much, in several ways at once.
* ''The Status Civilization'' by Creator/RobertSheckley. The protagonist is exiled to a planet of criminals as a convicted murderer. He doesn't believe he could have killed in cold blood but has to kill in self-defense in exile. In the end, he finds out the truth: [[spoiler:he was framed for a murder and he turned himself in because his subconscious considered him guilty and everyone on Earth was BrainWashed to turn themselves in.]]

* In the Literature/NewJediOrder series, after an attempt to brainwash her completely goes wrong, Tahiri ends up with one of these in the form of [[ScaryDogmaticAliens Yuuzhan Vong]] personality Riina Kwaad. Riina's not ''evil'' exactly, but she ''is'' very messed up and her attempts to pull a SplitPersonalityTakeover do a serious number on both their sanities. They end up doing a SplitPersonalityMerge instead.
* In ''Literature/TheHouseOfNight'', it usually involves Red Vampyres and Red Fledglings about how having dark feelings doesn't make you a monster, acting out on them does.
* ''Literature/DarkHeart'': The dark voice that bedevils Myrren during the journey to Corondor. At first it urges her to use her magic to kill, but eventually it starts fighting her for control of her own body. [[spoiler:It's eventually revealed to be her own demonic heritage which she's repressed for her entire life. She gets better once she comes to accept it and merges with it.]]
* In ''Literature/Babel17'' by Creator/SamuelRDelany, the person who has been sabotaging Rydra Wong's mission turns out to be [[spoiler:Rydra herself]], who had been secretly programmed by [[spoiler:simply learning the language Babel-17]], and was completely unaware of any of it.
* For werewolves in ''Literature/{{Newshound}}'', their lupine instincts act as this, constantly battling with the human mind for control and subtly influencing their thoughts and actions.
* ''Literature/JourneyToChaos'': After mana mutating into a monster, [[spoiler: Eric]] hears the monster's voice in his head, urging him to kill and eat every human he sees. He talks about its madness influencing his behavior to the point where he acts without thinking. Eventually, [[spoiler: he realizes that the mutation itself did not create a "monster side". All it did was loosen his natural inhibitions. He himself created the "monster side" subconsciously to distance himself from the event and deny the changes.]]
* In the ''Literature/ChaosGods'' series, Ki absorbs a demon at the end of the first book. It spends the next book trying to overpower her mind and take control of her body.
* Starting with ''The Beast that was Max'' by Gerard Houarner, this trilogy of horror novels features the middle-aged, sociopathic gov't assassin Max who's the "go to" man when it comes to dealing with especially dirty missions or missions involving the supernatural. That's Max at his nicest, inside of him is something he calls "The Beast". The Beast is the collection of all of Max's worst inclinations and made manifest, when Max is under the Beast's influence he's a total monster - he becomes a serial rapist, mutilates and tortures on a whim and etc. In one fight, while under the Beast's influence, he even almost gouges out the eyes of a team member. He does become separated from the Beast and both of them are trapped in a [[StrappedToAnOperatingTable horror hospital]] in Hell at the end of the 2nd novel.
* Subverted in ''Literature/RavellingWrath''. The setup of the story is that the Blood God will enter Rinn's mind, giving her magic powers and making her want to kill Yali. Which does happen... but when Rinn finally lets the Blood God fully take over near the end of the story, it leads to valuable communication that helps the heroes learn the true cause of the conflict.

* In ''Series/DoctorWho'', Cassandra's method of possession works like this. [[GrandTheftMe After snatching]] Rose's body, she explains Rose's persona is fully concious inside her head and can tell what's going on but can't do anything to control herself anymore. The effects of the psychograft are also slowly supressing her persona too, to the point that she would have been [[DeathOfPersonality completely assimilated by Cassandra's own personality eventually.]] She takes advantage of this in order to [[ShowingOffTheNewBody show off her new assets]] through [[ShamelessFanserviceGirl the entire plot]] in New Earth in order to get [[BestServedCold revenge on Rose]] and in order to continue living in a [[FantasticRacism young, "pure" human body]] and even [[BigDamnKiss gives The Tenth Doctor a passionate kiss.]] Although It's heavily implied she was being influenced by Rose's feelings.
* ''Series/{{Supernatural}}'''s Sam Winchester is a sort of human/demon hybrid and constantly fights against his demonic inheritance, which includes a portion of natural arrogance, but under the right circumstances ([[spoiler:drinking enough "demon blood"]]) turns him completely and murderously feral.
** Considering what transpires in the 6th season's finale [[spoiler:[[TheSoulless Soulless Sam]] likely counts as well]].
* The vampires from ''Series/BeingHumanUK'' seem to have shades of this, particularly Hal.

** Sort of applicable to Willow in Season 7. After her magical RoaringRampageOfRevenge as Dark Willow (her SuperPoweredEvilSide) at the end of Season 6, she's wary of using powerful magic for fear of losing control and turning evil again.

* ''Series/ForeverKnight'' has Nick Knight, a vampire trying to earn redemption as a Toronto homicide detective, struggling to suppress his predatory nature and bloodlust. Unlike the vampires in ''Series/{{Buffy|The Vampire Slayer}}'' and ''Series/{{Angel}}'', vampires in ''Forever Knight'' do generally have souls: most of them just give in to their new instincts fairly quickly and never look back.

* ''Series/KamenRiderWizard'': Phantom are monsters residing in the mindscapes of people with magical potential - Gates and work as a metaphor for how depression can destroy someone. Inducing a DespairEventHorizon in a Gate causes a Phantom to be born by tearing out of the victim's mind and ripping their body into shreds in the process.
* ''Series/KamenRiderExAid'': Inverted. Hiiro [[TeethClenchedTeamwork and]] Taiga thought that [[spoiler: Emu]]'s [[TheVirus Bugster]] is still inside and has to be removed first to cure him. As it turns out, they were very wrong as said [[DevilInPlainSight Bugster]] had become an EnemyWithout long time before that.
* ''Series/KamenRiderRevice'': Demons are created from negative emotions and common ills that plague human mind. They're harmless and inert unless released into a physical form and reality via a [[TransformationTrinket Vistamp]]. Then chaos ensues. [[spoiler: Daiji]]'s demon is a special case that only manifests by taking over his host, specifically targets his host's family and is always waiting in their shared mind for an oppurtunity to snatch control.
* Hyde from ''{{Series/Jekyll}}''. [[spoiler:Though he eventually stops being an enemy.]]
* Crichton when possessed by the Scorpius neural clone in ''Series/{{Farscape}}''.
* ''Series/{{CSI}}'' had a metaphorical version of this with Ray Langston and his two season struggle against the dark streak he felt he had. The UnstoppableRage variant is where it most likely fits. And it overtook him by the end of season 11 when [[spoiler:he killed Nate Haskell after Haskell tormented him and kidnapped his ex-wife.]]
* Possibly the trope namer, the ''Series/StarTrekTheOriginalSeries'' episode "[[Recap/StarTrekS1E5TheEnemyWithin The Enemy Within]]" features a TeleporterAccident which splits captain Kirk into a good/passive and an evil/aggressive version of himself.
* In the ''Series/StarTrekVoyager'' episode "[[Recap/StarTrekVoyagerS3E17Darkling Darkling]]", the Doctor tries to expand his program by incorporating personality aspects of various historical figures who possessed great minds. He failed to realize that he would also incorporate the darker sides of their psyches, and develops an evil split personality who takes Kes hostage.

* Towards the very end of ''Series/NightAndDay'', a black reptilian/humanoid monster begins to manifest itself – hiding under clothes, reflected in [[MirrorMonster mirrors]], stalking the catacombs or simply sitting at the Harpers' breakfast table. It’s never directly explained, but it seems to fulfil a symbolic function, representing the darker side of various characters, and perhaps humanity more generally.
* ''[[Series/TheOuterLimits1963 The Outer Limits]]'': In "The Man with the Power" a mild-mannered schoolteacher is implanted with a device that amplifies his brainwaves and gives him the power to control matter and energy directly. The rest of the episode has a very similar plot to ''Film/ForbiddenPlanet'', except that it takes place on contemporary Earth, and the resolution is the same: [[spoiler:The protagonist, riddled with guilt over the destruction he has caused, allows the power to destroy him and therefore itself]].
* In ''Series/Warehouse13'', [[spoiler: Ferdinand Magellan's Astrolabe allows the user to back 24 hours but also causes the user to develop a split personality whose action are to go against what was achieved through it (e.g Artie prevents [[TheEndOfTheWorldAsWeKnowIt society from collapsing]] but Evil Artie unleashes [[ThePlague a super virus]] instead)]].
* ''{{Series/Highlander}}'' had an episode with Michael Moore, an immortal who had a split personality. He doesn’t realize that he himself is Quentin Barnes, serial killer, until the end of the episode. Barnes takes control and Moore can only assert himself long enough to beg Duncan to take his head and end it.
* ''Series/MrRobot'': [[spoiler:The titular character is this to Elliot, forming fsociety and executing the war against E Corp behind his back.]]

* "Animal I Have Become" by Music/ThreeDaysGrace.
* "It's the Fear" by Music/WithinTemptation.
* "Demon Seed" by Music/NineInchNails
* "Innocence and Instinct" by Red is more or less an album about this.
* "Sad But True" by Music/{{Metallica}}
* "Sweating Bullets" by Music/{{Megadeth}}
* "Climbing Up the Walls" by Music/{{Radiohead}}
* "Monster" by Music/{{Skillet}}
* Arguably, "Down with the Sickness" by Music/{{Disturbed}}
* "Behind Blue Eyes" by Music/TheWho
* "[[ExactlyWhatItSaysOnTheTin Enemy Within]]" by Arch Enemy
* "The Enemy Inside" by Music/DreamTheater.
* "World War Me" by Theory of a Deadman.
* The Idea behind ASP's first five albums (The Black Butterfly cycle), especially [[https://www.youtube.com/watch?v=w6z3MRyz5UQ Die kleine Ballade vom schwarzen Schmetterling]]
* Music/PoetsOfTheFall's "[[https://youtu.be/xfvtPTvuFTA Drama for Life]]" has the singer doing [[BattleInTheCenterOfTheMind battle]] with his rampant GhostInTheMachine before deciding to TakeAThirdOption and "embrace" its chaotic influence.
* [[Music/{{Eminem}} Slim Shady]] is this for Marshall Mathers. Eminem first took on the character with a [[KayfabeMusic kayfabe]] SplitPersonalityTakeover, and has touched on this theme in several songs. It's especially prominent in "My Darling", in which Slim Shady - in his evil spirit form - persuades Marshall that his career is dependent on him accepting him again, and takes over him as the revving chainsaw in the beat builds up into a whine.

* OlderThanPrint: Werewolves in any story in which the werewolf form is disconnected from, or uncontrollable by, the human form. There are sympathetic folktales about otherwise good people suffering from lycanthropy.

* ''Roleplay/FireEmblemOnForums'':
** ''Roleplay/FireEmblemOnForumsDemonSoulSaga'': The Inner Demon, a manifestation of one's own emotions, is one of these that resides in ''every single person on the planet''. However, they only wake up when exposed to an Akuma Seed, whereupon they begin trying to take over their host. If they succeed, they create an Akuma; if they fail, they instead create a Kaijin, comprising most of the protagonists.

* At the risk of too much JustForFun/TropeOverdosed, ''TabletopGame/Warhammer40000''. [[TheCorruption Chaos]] is always recruiting.
** To emphasize this, Imperial propaganda reminds you that [[EverythingTryingToKillYou "For every enemy without there are a hundred within"]]. They're talking about the Imperium itself, but what that means is that it's ''teeming'' with heretics, so many that they outnumber those actually loyal to the Emperor a billion to one. Regardless of how much it's emphasized, there's a bit a truth to it: Warhammer 40,000 has enemies within in spades, and all too often the enemy within wins.
*** Certain factions requires their members to conquer their enemy inside for them to join fully. The Space Wolves initiation rites have a phase in which you must conquer and learn to manage to maintain your beast within, or you'll turn into a Wulfen, a werewolf-like being. And even once you have managed to pass this stage, it is not sure whether or not the curse is truly gone. Already initiated members can have the curse activate in the middle of the battlefield, and gain strength beyond even their already superhuman abilities. While this is usually temporary and the one affected will return to his normal form after the battle is over, this is the result of the curse not stabilizing completely during the initiation rites.
** Eldar warriors must contain the curse of Khaine within them, learning to don and doff their 'war mask' on command, lest they be trapped upon the Path of the Warrior and become an Exarch, forever imprisoned within the armor of his predecessors, bound into the gestalt consciousness that is the Exarch, [[AndIMustScream incapable of peace and unable to die]].

* ''TabletopGame/VampireTheMasquerade'' and ''TabletopGame/VampireTheRequiem'' have a large part of their mythoi based around this - each Vampire is at least partially under the control of "The Beast," the representation of their HorrorHunger. The Beast is their id; it makes them feed, it makes them run from fire and oncoming sunlight, and it makes them [[UnstoppableRage Frenzy]] if they get scared or hungry enough. ''Masquerade'' has the [[ArcWords arc phrase]] "A beast I am, lest a Beast I become": Try not to harm mortals period, and you'll ultimately become a far worse monster than those who are willing to kill and feed in moderation.
* ''TabletopGame/WraithTheOblivion'' has a similar threat in the form of Shadows. When a person dies and becomes a wraith, the unconscious urge for death merges with all the wraith's regrets, doubts, and self-hatred to form something plugged into the very depths of Oblivion. It wants the wraith to basically give up on everything that defined them in life, admit that it's all meaningless, and join the great crusade to pull the plug on existence forever and ever. Some constantly harangue the wraith, while others try to offer "helpful" advice to ensure survival in the Shadowlands.
* In ''TabletopGame/DungeonsAndDragons'' 4th Ed the Minotaurs have to struggle against releasing the beast within. So much that they adorn everything they wear and carry, and sometimes themselves, with labyrinth designs to symbolize that struggle in their mind and soul.

* Emil, the main character from ''VideoGame/TalesOfSymphoniaDawnOfTheNewWorld'', is a strange case. As it stands at the beginning, he has the original flavor: a bad case of multiple personality disorder, in which his usually meek self is replaced with a powerful, badass red-eyed alter-ego whenever he needs to fight. Later this side of him becomes increasingly dominant and much more of a jerk. While at first it appears it's a part of his contract with Ratatosk, we later find out that Emil is really [[spoiler:being possessed by Ratatosk himself, the sentient-life-hating deity, that the supposed antagonist Richter has been trying to kill all along. But then comes the BIG twist: "Emil" really IS Ratatosk, amnesiac after being awakened improperly. In order to fill in the gaps in his memory, Ratatosk created a new personality for himself, styling his appearance after Richter's best friend Aster, whose murder started the whole debacle, and taking the name "Emil" from a random dying woman in Palmacosta who didn't recognize him in her dying moments and mistook Ratatosk for her son. So the SplitPersonalityTakeover in this case is the ''original'' persona reasserting dominance over the facade it had created.]]
* The protagonist of ''VideoGame/WildArms2'' fuses with a demon destined to destroy the world and because of this he can use the sword destined to defeat said demon...on other villains.
* Both Leona & Iori from ''VideoGame/TheKingOfFighters'' series struggle with the curse of their bloodlines, which at times can cause them to regress into a [[UnstoppableRage berserker state]] know as "Riot of the Blood". Leona killed her entire family when she was in the Riot, and Iori has killed or severely wounded others as well.
* In TheVeryDefinitelyFinalDungeon of ''VideoGame/PlanescapeTorment'', the BigBad traps your mind inside a magic prison, where the personalities of three of your most powerful prior incarnations -- [[AxeCrazy The Paranoid Incarnation]], [[ManipulativeBastard The Practical Incarnation]], and [[TheAtoner The Good Incarnation]] -- [[AmnesiacDissonance splinter from your mind]] and start interacting with you. Reintegrating with your body is only possible through subduing or uniting all four fragments until [[ThereCanBeOnlyOne only one remains]], and the Practical Incarnation isn't particularly well disposed towards [[GrandTheftMe letting]] ''[[GrandTheftMe you]]'' [[GrandTheftMe be the one to do this]]. In fact, it's implied that being able to resurface and take over your body has been part of his plan all along -- a means of surviving beyond death, if his initial plan to defeat the evil had failed.
* In ''VideoGame/BaldursGateII'', the main character ultimately is this, right from the plot premise: [[spoiler:he/she is the child of the now deceased God of Murder, and the recurrent theme is whether he/she will suppress the taint in his/her soul or embrace it.]] He or she then will also learn a new power: [[spoiler:the ability to turn into a powerful beast, the Slayer, at the price of risking his/her own life if the transformation lasts too long (the game will promptly send a warning, with the dialogue menu telling that the main character feels that he/she's losing control)]].
* [[spoiler:Akachi]] in ''VideoGame/NeverwinterNights2: Mask of the Betrayer'' forces you to continually consume other spirits or have your soul consumed instead. The main quest of the game is figuring out a way to get rid of the curse. [[spoiler:The most extreme Evil ending actually involves you ''devouring Akachi's spirit itself'' to become a [[EldritchAbomination horrific, god-killing abomination.]]]]
* In ''VideoGame/PrinceOfPersiaTheTwoThrones'', the Prince's arm is injured and subsequently corrupted by the Sands Of Time. As this taint slowly kills him his own darker personality begins to manifest as the "Dark Prince."
** The Dark Prince initially acts like a benign ExpositionFairy, seeming to be just like the Prince, only with fewer scruples. Over the course of the story the corruption caused by the sands grows worse and the instances of the darker personality coming to the surface becomes more frequent. However whilst his outward appearance changes to that of the Dark Prince when overwhelmed by the sands taint, he retains enough control to seek out water to keep the curse at bay. Failing to reach water in time during these sequences causes [[NonStandardGameOver him to die]] due to the corruption overwhelming him.
** Strangely enough for this trope, he never makes an actual bid for control over the Prince's body until the very end, instead spending most of his time trying to [[TheCorrupter corrupt the Prince]]. Though until the Prince had managed to suppress the Dark Prince through [[HeroicWillpower willpower]] alone, he had little reason to vie for control as he was certain the corruption would kill him in due time. After the Prince rejects his help, he just petulantly taunts and distracts you during ThatOneLevel.
* ''VideoGame/Condemned2Bloodshot'' has the Alcohol Demon, a manifestation of Ethan Thomas' alcoholism. At numerous occasions through the game, Ethan is tormented by the damn thing [[spoiler:until he eventually "kills" it, in a bar no less. He was, however, trying to help you get yourself together. It doesn't die - it comes back a mission or two later, and points out it represents ''all'' his demons, not just alcohol.]]
* The final boss in ''VideoGame/CastlevaniaAriaOfSorrow'' is the embodiment of Dracula's evil, which threatens to possess and overwhelm Dracula's reincarnated self, [[spoiler:Soma Cruz.]]
* ''Franchise/{{Persona}}s'' are supernatural spirits that exist within a person's psyche, representing their unconscious selves. Hence, the more nasty Personas tend to have unpleasant effects on their host.

* ''VideoGame/{{Xenogears}}'': Fei and [[spoiler:[[SuperpoweredEvilSide Id.]] Subverted however when it turns out that it's arguable Id is the real personality.]]
* In ''VideoGame/Sly3HonorAmongThieves'', The Panda King (one of the villains of the first game) is trapped within his mind of his failure against Sly. Eventually Sly himself enters his mind to convince him into an EnemyMine and snap out of the trance to save his daughter, who's captured by a local warlord.
* A rather weird example PlayedForLaughs is found at the end of ''VideoGame/FalloutNewVegas'' DLC ''Old World Blues'', where you confront [[spoiler:your very own BrainInAJar, who is completely pissed off at you for the various wacky hijinks you've been getting into back in the Mojave at the expense of your body and hygiene. You then have to convince it to cooperate with you and if you want, rejoin your body]]. With the appropriate trait, you can [[ScrewYourself hit on your own brain]] in that scene.
* A variation is used in ''VideoGame/MetalGearSolidPortableOps''. [[spoiler:After the ICBMG is completed, Gene implies in his speech that there is a soldier under his employ that is ordered to kill them for their "betrayal." He then throws a knife at the unfortunate soldier that catches he eye, and the stabbing pain results in him firing his rifle at another soldier unwillingly. The soldiers then fire amongst themselves.]]

* Pyramid Head from ''Franchise/SilentHill'' is the physical manifestation of [[spoiler:James Sunderland]]'s desire to be punished for [[spoiler:murdering his wife]]. By the end of the game, when he come to terms with his own guilt, [[spoiler: Pyramid Head kills himself]].
* In ''VideoGame/EarthBound1994'', [[spoiler:Ness's Nightmare is the manifestation of his inner demons within Ness's mind that holds back his full power. After Ness defeats it, his PsychicPowers expand rapidly]].

** Shiki Tohno has one in the form of Shiki Nanaya. He is the embodiment of the bloodthirsty, sadistic killer that Shiki fears he could become (or would have become had his life gone a different direction), the one that [[HealingFactor almost killed]] [[LoveInterest Arcueid]].
** [[FriendlyNeighborhoodVampire Arcueid]] has her "[[MonsterProgenitor father]]" the Crimson Moon, who's considering using her as a vehicle for his resurrection. In ''Kagetsu Tohya'', it's revealed that [[HunterOfMonsters Ciel]] has her own evil past self she fears, who manifests the same way Nanaya does; and back in the original work, everyone who gets possessed by [[BigBad Roa]] develops him as an Enemy Within, including [[spoiler:Shiki, giving him two simultaneous enemies within.]]
* In ''VideoGame/{{Disgaea 3|AbsenceOfJustice}}'', Dark Mao is Mao's pure evil essence embodied into a Heart Resident. [[spoiler:Dark Mao was being secretly raised by Aurum during his time as Geoffrey to help turn Mao into the "Strongest Overlord".]]
* In the final part of the bonus chapter of ''Nevertales: The Beauty Within'' the main character's husband has to fight a physical representation of the beast inside of him in order to control his rage.
* In the ''Franchise/{{Tekken}}'' franchise, protagonist Jin Kazama is plagued by the influence of the Devil Gene passed down through his family's bloodline. His father, Kazuya Mishima, also has it, and eventually came to embrace it, purging himself of all good in him. Jin fights to rid himself as well as the whole world of the Devil Gene. [[spoiler:Eventually, he uses the army of his family company to lure out the very source of the gene, [[GodOfEvil Azazel]], and uses his own Devil power to destroy him. It didn't work. He has to find the real source of the Devil Gene: His grandmother.]]
* [[BrainwashedAndCrazy Widowmaker]] from ''VideoGame/{{Overwatch}}'' is an inverted example, as it's shown by a few of her lines that, despite being brainwashed into a sadistic assassin, the old Amélie is still somewhere inside, aware of what she's done.
* ''Franchise/{{Danganronpa}}'' has [[spoiler:Touko Fukawa, a somewhat misanthropic, shy {{Bookworm}} with something of a persecution complex who, up until chapter three, looks like she couldn't hurt a fly if she wanted to...until it's revealed that she has a SplitPersonality, a bloodthirsty LargeHam SerialKiller named Genocider Syo that Fukawa's mind developed to help cope with the trauma caused by her [[ParentalNeglect abusive, neglectful parents]] and all the repressed emotions that came with it. Fukawa does everything in her power to keep Syo at bay, and that includes not getting close to anyone, because Syo loves to kill the boys that she thinks are attractive, lest Syo kills again and [[KillTally carves another tally mark into Fukawa's thigh.]] Despite her best efforts, Syo does come out against Fukawa's will whenever she either sneezes or [[AfraidofBlood sees blood and faints]]]].
* In ''Franchise/TheElderScrolls'', Sheogorath, the [[OurGodsAreDifferent Daedric Prince]] of [[MadGod Madness]], has an ArchEnemy in Jygglag, the Daedric Prince of [[ControlFreak Order]] who comes at the end over every Era in an event known as the Greymarch and destroys Sheogorath's realm, the [[EldritchLocation Shivering Isles]]. In the eponymous expansion for ''[[VideoGame/TheElderScrollsIVOblivion Oblivion]]'', it is revealed that [[spoiler:Sheogorath and Jyggalag are one in the same. Sheogorath is the [[SealedInsideAPersonShapedCan Person Shaped Can]] that Jyggalag was sealed within by the other Daedric Princes, who were fearful of his growing power. The main plot of the expansion sees you end this ViciousCycle and free Jyggalag, by [[AGodIsYou taking the mantle of Sheogorath for yourself]]]].
* In ''VideoGame/WorldOfWarcraft'' [[DemonSlaying demon hunters]] are elven warriors who absorb demon souls to gain superior combat abilities... and an inner demon that tries to break free and gain control over their body. Demon hunters must always control it using magical tatoos and willpower.
* ''VideoGame/CaptiveRPGMaker'': Implied- the protagonist has a voice inside her head that will pop up and occasionally stop her from trying to escape the basement by telling her things like "DON'T LEAVE". [[spoiler:The Remember ending implies her to be the manifestation of the VillainProtagonist's memories pre-amnesia, who tells her to "FINISH THE JOB" and kill the FinalGirl.]]
* ''VideoGame/BatmanTheTelltaleSeries''' first season had Harvey Dent [[EvilMeScaresMe trying to keep Two-Face's influence at bay]], while the second season was even ''named'' "The Enemy Within", reflecting [[spoiler: John Doe's struggles against his violent urges]].

[[folder:Web Comics]]

* The Other (aka [[spoiler:Lucrezia Mongfish]]) from ''Webcomic/GirlGenius'' stays as a GreaterScopeVillain until Volume 5, when a copy of it's personality is downloaded into the mind of [[spoiler:Agatha Heterodyne, the main heroine. By the end of Vol. 6, Agatha regains her special locket that is able to hold back The Other's power over her body, but throughout the rest of the comic The Other is still pushing inside Agatha's mind for control]]. This situation remains until [[spoiler:Volume 20 when the copy is ''finally'' removed from her mind]].

* In ''Webcomic/MobPsycho100'', the last arc is [[spoiler:Mob’s internal fight against ???%, the personafication of everything he feared about himself.]]
* ''Webcomic/TheOrderOfTheStick'' has Durkon get turned into a vampire, and initially he seems to have retained his personality and is still loyal to the team, but TheReveal is that Vampire Durkon is actually this. The real Durkon is trapped inside his own head while a negative energy spirit created from all the emotions he felt on his worst ever day is in the driver's seat. The vampire has all of Durkon's anger and resentment, but doesn't actually have the memories of the events that caused them; he uses Durkon as a living memory bank, calling up any knowledge he needs to further his ruse, as he betrays the team and plots to end the world. Durkon is eventually able to trick the vampire into viewing a particularly traumatic and confusing memory (learning that the reason he and his mother lived in poverty is because she gave away a fortune to have some strangers resurrected), which baffles the vampire, who then demands Durkon's ''positive'' memories as a way to cope with the confusion. Turns out giving your Enemy Within the good along with the bad can have [[https://www.giantitp.com/comics/oots1130.html extreme results]].

[[folder:Web Original]]
* Yoi, of ''Literature/MagicalGirlHunters'' has his split personality [[spoiler:Koi]], who eventually gains his own body.
[[/folder]]

* TheReveal behind the BigBad in ''WebVideo/{{Animerica}}'''s second season is [[spoiler:that he has a "demon side", being created from his constantly repressed negative thoughts on his twin Kasuse and the fear and sadness he felt from being TheUnfavorite. The evil side constantly makes use of MoreThanMindControl to his good side, promising that EvilFeelsGood and that he'll have a better life. However by Season 3 when the two converse once more Kiyone rejects his evil side, declaring that it only made his life worse and that he wants to [[TheAtoner atone]] for his sins. It is due to [[YouAreNotAlone Ron's speech]] and his [[ThePowerOfLove love for Yumi]] that he manages to overcome the demon within him.]]

* ''WesternAnimation/AdventureTime'': The Lich is this to Sweet P until the latter kills his Farmworld Copy, after which Sweet P seems to have fully overcome his influence.
* ''WesternAnimation/DuckTales2017'': In "The Split Sword of Swanstantine!", Lena enters Huey's mindscape and discovers a metal door containing "The Duke of Making a Mess", a frazzled, savage version of Huey that serves as the personification of [[EmotionSuppression the extreme emotions he wants to keep at bay]].
* Jack's Inner Self [[spoiler:aka Mad Jack]] from ''WesternAnimation/SamuraiJack'' ([[SamuraiJack/TropesSeason5 season 5]]). [[spoiler:How Jack had become virtually emotionless and cold to the point of simply abandoning hope and relinquishing himself to despair in Season 5? That'd be the result of this side of him, driving his actions in the interest of simply doing all he could to survive.]]

** Beast Boy briefly develops one in the episode appropriately titled "The Beast Within". After getting exposed to strange chemicals, he briefly starts acting [[NotHimself rude and confrontational]], then develops a super-powered "were-beast" form (although said beast wasn't ''quite'' as dangerous as it first appeared). He and Raven actually have a little chat about this at the end of the episode.
* Season two of ''WesternAnimation/LegionOfSuperHeroes2006'' sees Brainiac 5 discover the power available by accessing evil Brainiac 1.0's memories. But then, well, ''this trope''. He ends up playing super-TabletopGame/{{chess}} in his head.
* PlayedForLaughs in ''WesternAnimation/RickAndMorty'' when Rick buys a device that allows the family's [[ShadowArchetype subconscious minds]] to take control of their bodies and do their chores while they sleep. Because of Rick's mistreatment, they eventually go rogue and plot a SplitPersonalityTakeover.
* In ''WesternAnimation/ScoobyDooMysteryIncorporated'', it turns out that association with the Nibiru entity inverts this trope by slowly sealing away someone's good side in a frozen extradimensional room. Hence Nibiru is responsible for all the other villains in the show.
* Season 3 of ''WesternAnimation/StarTrekLowerDecks'' reveals that [[spoiler:Rutherford used to be an egotistical, sleazy jerk]] who was helping a shady superior officer on a clandestine project. [[spoiler:The reason for his bouts of ill-temper since his implant's replacement is the old Rutherford trying to resurface]]. It culminates in a BattleInTheCenterOfTheMind that the old personality loses--and declining to merge with the newer, nicer version because it would be like killing them both.
* Mike from ''WesternAnimation/TotalDrama'' has his [[spoiler:[[SplitPersonality hidden alternate personality]] [[TheDreaded Mal]] in "All-Stars"]].
* Korra from ''WesternAnimation/TheLegendOfKorra'' underwent this due to the PTSD she received as a result of the near death experience at the end of the third season. She frequently saw a version of herself in the Avatar State with the same appearance that she had in the finale. It haunts her throughout Season 4, to the point it stops her dead in her tracks. Also, it isn't limited to the mind either, and has attacked Korra in the physical world too. [[spoiler: Though the series implies this is more her self-doubt from being beaten so badly at the end of season 3. She isn't so much fighting a spirit as fighting herself to regain her balance.]]
* ''WesternAnimation/{{WITCH}}'': [[spoiler: After being kidnapped by [[BigBad Nerissa]], [[UnderstandingBoyfriend Matt]] gets a Demon of Hate put inside him, turning him into [[WickedWitch Nerissa's]] [[TheDragon Dragon]]. ]] At least he gets a cool mask and a wicked FallenAngel look.
* ''WesternAnimation/TheOwlHouse'': The Owl spirit is this to Eda as it'll come out and morph her body into a feral creature if she doesn't take a specialized elixir to suppress it. As the series goes on, its threat diminishes a bit as Luz and King manage to somewhat tame the creature to at least recognize them as friends. [[spoiler: Later on Eda actually meets the spirit within her body and finds out it really just a scared creature, confused at what's going on. Similar to ''Naruto'', Eda befriends it rather then suppress it. In doing so, this allows her a power up where she's in control of her body.]]

* White Court Vampires in ''Literature/TheDresdenFiles''. It's called their Demon and their Hunger, and only Thomas even tries to fight it. It exerts control especially in the presence of the relevant emotional "food", though if the vampire tries to starve it out they go mad with Hunger and mindlessly try to Feed. It can be killed if the host experiences the true emotion related to their feeding - love vs lust, etc.

* ''Manga/{{Naruto}}'': The Nine-Tailed Fox was initially this for Naruto in the early parts of the manga if he tapped too much into his power, running the risk of overtaking Naruto, being released and running rampant, to the point the fox even urging Naruto to use more of his power when in fights he was losing badly in. [[spoiler: It's only in the second half that Naruto realizes the fox spirit is more a victim just wanting to avoid being controlled again by Madara and the two eventually agree to work together, becoming friends and allies in the process that negates this threat.]]

* Season 3 of ''WesternAnimationStarTrekLowerDecks'' reveals that [[spoiler:Rutherford used to be an egotistical, sleazy jerk]] who was helping a shady superior officer on a clandestine project. [[spoiler:The reason for his bouts of ill-temper since his implant's replacement is the old Rutherford trying to resurface]]. It culminates in a BattleOfTheCenterOfTheMind that the old personality loses--and declining to merge with the newer, nicer version because it would be like killing them both.

!!Examples

** In [[VideoGame/KingdomHeartsI the first game]], RivalTurnedEvil Riku is convinced/tricked into fusing with the BigBad, losing control of his body in the process. He redeems himself, but is struggling with the darkness within in the sequels. This is played up the most in ''VideoGame/KingdomHeartsChainOfMemories'', where Ansem makes more than one attempt to retake control.
** Inverted when [[spoiler:Xehanort took possession of Terra's body. During the fight with Aqua, Terra's heart reached out to hinder Xehanort's movement, prompting the guy to cut his losses and lock up both hearts' memories. The same thing happens a decade later, when Riku stops "Ansem" from approaching Kairi from the inside.]]

* The Personas (of the ''Franchise/{{Persona}}'' RPG series) are supernatural spirits that exist within a person's psyche, representing their unconscious selves. Hence, the more nasty Personas tend to have unpleasant effects on their host.

* ''VideoGame/{{Xenogears}}'', Fei and [[spoiler:[[SuperpoweredEvilSide Id.]] Subverted however when it turns out that it's arguable Id is the real personality.]]

* In the ''VideoGame/{{Tekken}}'' franchise, protagonist Jin Kazama is plagued by the influence of the Devil Gene passed down through his family's bloodline. His father, Kazuya Mishima, also has it, and eventually came to embrace it, purging himself of all good in him. Jin fights to rid himself as well as the whole world of the Devil Gene. [[spoiler:Eventually, he uses the army of his family company to lure out the very source of the gene, [[GodOfEvil Azazel]], and uses his own Devil power to destroy him. It didn't work. He has to find the real source of the Devil Gene: His grandmother.]]

[[folder:Webcomics]]

Not to be confused with the expansion pack to the ''[[VideoGame/XComEnemyUnknown X-Com]]'' reboot, nor an unrelated [[Series/EnemyWithin TV series]], or another unrelated [[Literature/{{Hitman}} novel]]. Contrast EnemyWithout. Compare JekyllAndHyde. Also not to be confused with KillItThroughItsStomach, which involves an enemy ''literally'' inside another. Usually part of their [[AnatomyOfTheSoul Soul Anatomy]].

* In the spinoff AU for ''Roleplay/TheStrexFamily'', ''Compliance'', [[spoiler:Carlos]] has an inner voice goading him to kill his captors in a variety of painful ways. [[spoiler:Compliance ends with the WhamMoment that the voice took over during the climactic scene, and for the entirety of the sequel, ''Procedure'', he's [[JumpingOffTheSlipperySlope jumped off the slippery slope]] and let the EnemyWithin control him.]]

* Possibly the trope namer, the episode [[{{Recap/StarTrekS1E5TheEnemyWithin}} The Enemy Within]] of ''Series/{{Star Trek|The Original Series}}'' features a transporter malfunction which splits captain Kirk into a good/passive and an evil/aggressive version of himself.
* In an episode of ''Series/StarTrekVoyager'', the Doctor tries to expand his program by incorporating personality aspects of various historical figures who possessed great minds. He failed to realize that he would also incorporate the darker sides of their psyches, and develops an evil split personality who takes Kes hostage.

** [[FriendlyNeighborhoodVampire Arcueid]] has her "[[MonsterProgenitor father]]" the Crimson Moon, who's considering using her as a vehicle for his resurrection. In ''Kagetsu Tohya'', it's revealed that [[HunterOfMonsters Ciel]] has her own evil past self she fears, who manifests the same way Nanaya does; and back in the original work, everyone who gets possessed by [[BigBad Roa]] develops him as an EnemyWithin, including [[spoiler:Shiki, giving him two simultaneous enemies within.]]

A specific form of SplitPersonality. Maybe the BodyHorror became a bit ''too'' fused with someone. Maybe the UnstoppableRage is getting... too unstoppable. Perhaps TheAtoner's past is taking on a life of its own. A [[HalfHumanHybrid half-human's]] [[VillainousLineage Demon]] / [[{{Dhampir}} Vampire self]] is trying to take control, or that DealWithTheDevil is hitting a bit too close to home.

* The eponymous character of ''Manga/SoulEater'' deals with this, especially as it pertains to keeping Maka safe. He has a small ogre-demon living inside his soul. It starts to take over in the final battle with Asura (in the anime), when Maka has to [[spoiler:go into Soul's soul and save the real Soul from the ogre-demon, who has disguised himself to look exactly like the guy]], in order to save her partner from the Madness with the PowerOfLove.

* ''Manga/{{Naruto}}'': The Nine-Tailed Fox was initially this for Naruto in the early parts of the manga if he tapped too much into his power, running the risk of overtaking Naruto, being released and running rampant. Even to the point the fox urging Naruto to use more of his power when in fights he was losing badly in. [[spoiler: It's only in the second half that Naruto realizes the fox spirit is more a victim just wanting to avoid being controlled again by Madara and the two eventually agree to work together, becoming friends and allies in the process that negates this threat.]]

** In [[VideoGame/KingdomHeartsI the first game]], RivalTurnedEvil Riku is convinced / tricked into fusing with the BigBad, losing control of his body in the process. He redeems himself, but is struggling with the darkness within in the sequels. This is played up the most in ''VideoGame/KingdomHeartsChainOfMemories'', where Ansem makes more than one attempt to retake control.

* On a very similar show, season two of ''WesternAnimation/LegionOfSuperHeroes'' saw Brainiac 5 discover the power available by accessing evil Brainiac 1.0's memories. But then, well, ''this trope''. He ends up playing super-TabletopGame/{{chess}} in his head.

* ''TabletopGame/VampireTheMasquerade'' and ''TabletopGame/VampireTheRequiem'' have a large part of their mythoi based around this - each Vampire is at least partially under the control of "The Beast," the representation of their HorrorHunger. The Beast is their id; it makes them feed, it makes them run from fire and oncoming sunlight, and it makes them [[UnstoppableRage Frenzy]] if they get scared or hungry enough. ''Masquerade'' has the [[ArcWords arc phrase]] "A beast I am, lest a Beast I become": Try to be humane and not harm mortals, and you'll ultimately become a far worse monster than those who are willing to kill and feed in moderation.

* Ryner Lute from ''LightNovel/TheLegendOfTheLegendaryHeroes''. The three inhabitants of his mind include a being that results in a SuperpoweredEvilSide, a generally benevolent being, and [[spoiler:his mother]], the last of whom exists to try and counteract the first one.

** [[FriendlyNeighborhoodVampire Arcueid]] has her "[[MonsterProgenitor father]]" the Crimson Moon, who's considering using her as a vehicle for his resurrection. In ''Kagetsu Tohya'', it's revealed that [[HunterOfMonsters Ciel]] has her own evil past self she fears, who [[YourWorstNIghtmare manifests the same way Nanaya does]]; and back in the original work, everyone who gets possessed by [[BigBad Roa]] develops him as an EnemyWithin, including [[spoiler:Shiki, giving him two simultaneous enemies within.]]

-->-- '''Music/{{Rush}}''', "The Enemy Within"

* ''Webcomic/TheBeastLegion'': Fyre [[http://www.thebeastlegion.com/comic/issue-12-page-05-high-stakes engages in a battle with her own inner rage]] that is intent on destroying her personality & taking over...

* ''Webcomic/GirlGenius'': The Other (aka [[spoiler:Lucrezia Mongfish]]) stays as a GreaterScopeVillain until Volume 5, when a copy of its personality is downloaded into the mind of [[spoiler:Agatha Heterodyne, the main heroine. By the end of Vol. 6, Agatha regains her special locket that is able to hold back The Other's power over her body, but throughout the rest of the comic The Other is still pushing inside Agatha's mind for control]]. This situation remains until [[spoiler:Volume 20 when the copy is ''finally'' removed from her mind]].
%%* ''Webcomic/InWilysDefense'': [[spoiler:Sputnik for Cutman]].
* ''Webcomic/MobPsycho100'': The last arc is [[spoiler:Mob's internal fight against ???%, the personafication of everything he fears about himself]].

%%* ''Webcomic/TwoKinds'': Trace Legacy [[spoiler:and his previous self, before losing his memories]] is one of these, though he's not actively fighting it... yet.
* ''Webcomic/ZebraGirl'': Mr. Chalk seems to be the manifestation of Sandra's demonic taint and appears in her dreams in order to drive her to cause pain and suffering. Amazingly, he's ''not'' the one that caused her FaceHeelTurn.

** The Mantle of the Winter Knight is this for its host. It's a SuperEmpowering presence bestowed onto humans, and it holds a primitive, selfish intelligence that tries to push its host into becoming a more monstrous individual to suit it's own desires. It's previous host, a man named Lloyd Slate, was rather infamous in his circle for being a lying, murdering rapist and heroin addict.

* ''VideoGame/{{Xenogears}}'', Fei and [[spoiler:[[SuperpoweredEvilSide Id]] Subverted however when it turns out that it's arguable Id is the real personality.]]

* ''Webcomic/GirlGenius'': The Other (aka [[spoiler:Lucrezia Mongfish]]) stays as a GreaterScopeVillain until Volume 5, when a copy of it's personality is downloaded into the mind of [[spoiler:Agatha Heterodyne, the main heroine. By the end of Vol. 6, Agatha regains her special locket that is able to hold back The Other's power over her body, but throughout the rest of the comic The Other is still pushing inside Agatha's mind for control]]. This situation remains until [[spoiler:Volume 20 when the copy is ''finally'' removed from her mind]].

* ''Webcomic/TheOrderOfTheStick'' has Durkon get turned into a vampire, and initially he seems to have retained his personality and is still loyal to the team, but TheReveal is that Vampire Durkon is actually this. The real Durkon is trapped inside his own head while a negative energy spirit created from all the emotions he felt on his worst ever day is in the driver's seat. The vampire has all of Durkon's anger and resentment, but doesn't actually have the memories of the events that caused them; he uses Durkon as a living memory bank, calling up any knowledge he needs to further his ruse, as he betrays the team and plots to end the world. Durkon is eventually able to trick the vampire into viewing a particularly traumatic and confusing memory (learning that the reason he and his mother lived in poverty is because she gave away a fortune to have some strangers ressurected), which baffles the vampire, who then demands Durkon's ''positive'' memories as a way to cope with the confusion. Turns out giving your Enemy Within the good along with the bad can have [[https://www.giantitp.com/comics/oots1130.html extreme results]].

* ''WesternAnimation/TheOwlHouse'': The Owl spirit is this to Eda as it'll come out and morph her body into a feral creature if she doesn't take a specialized elixir to suppress it. As the series goes on, it's threat diminishes a bit as Luz and King manage to somewhat tame the creature to at least recognize them as friends. [[spoiler: Later on Eda actually meets the spirit within her body and finds out it really just a scared creature, confused at what's going on. Similar to ''Naruto'', Eda befriends it rather then suppress it. In doing so, this allows her a power up where she's in control of her body.]]

* ''Manga/{{Naruto}}'': The Five Tailed Fox was initially this for Naruto in the early parts of the manga if he tapped too much into his power, running the risk of overtaking Naruto, being released and running rampant. Even to the point the fox urging Naruto to use more of his power when in fights he was losing badly in. [[spoiler: It's only in the second half that Naruto realizes the fox spirit is more a victim just wanting to avoid being controlled again by Madara and the two eventually agree to work together, becoming friends and allies in the process that negates this threat.]]

* In ''Webcomic/TheBeastLegion'', Fyre [[http://www.thebeastlegion.com/comic/issue-12-page-05-high-stakes engages in a battle with her own inner rage]] that is intent on destroying her personality & taking over...

* The Other (aka [[spoiler:Lucrezia Mongfish]]) from ''Webcomic/GirlGenius'' stays as a GreaterScopeVillain until Volume 5, when a copy of it's personality is downloaded into the mind of [[spoiler:Agatha Heterodyne, the main heroine. By the end of Vol. 6, Agatha regains her special locket that is able to hold back The Other's power over her body, but throughout the rest of the comic The Other is still pushing inside Agatha's mind for control]]. This situation remains until [[spoiler:Volume 20 when the copy is ''finally'' removed from her mind]].
%%* [[spoiler:Sputnik for Cutman]] from ''Webcomic/InWilysDefense''.
* In ''Webcomic/MobPsycho100'', [[spoiler: the last arc is Mob’s internal fight against ???%, the personafication of everything he feared about himself.]]

-->'''Durkon:''' But be careful. B'cause ye know wha ye are if'n ye haf me body an' all me joys an' sorrows?
-->'''Durkon:''' Yer me.
%%* In ''Webcomic/TwoKinds'' Trace Legacy [[spoiler:and his previous self, before losing his memories]] is one of these, though he's not actively fighting it... yet.
* Mr. Chalk from ''Webcomic/ZebraGirl'', seems to be the manifestation of Sandra's demonic taint and appears in her dreams in order to drive her to cause pain and suffering. Amazingly, he's ''not'' the one that caused her FaceHeelTurn.

* Rand al'Thor and Lews Therin Telamon in Robert Jordan's ''Literature/TheWheelOfTime'' series. It's never made certain whether Lews Therin really does exist, or whether Rand is simply becoming insane due to the taint on Saidin and pure stress. In the eleventh book, Semirhage [[spoiler:reveals that those who hear the voices of previous incarnations are doomed to go mad anyway and is in fact worse as, unlike with ''merely'' insane people, there is little hope of recovery because the voices are real after all.]]
** Given the number of things he could not know unless the voice in his head really is Lews Therin, it's pretty certain. It also shows up long before he develops any of his other Taint-related symptoms.
** It is now confirmed [[spoiler:that Rand had access to Lews Therin's memories because he is his reincarnation, but the actual Lews Therin "voice" was just a symptom of his madness and identity issues. He appears to have sorted that out with a SplitPersonalityMerge as of the]] end of the 12th book.

** In an impressive and rare display, by ''[[VideoGame/KingdomHeartsDreamDropDistance Dream Drop Distance]]'' the Big Bad has ''given up'' on trying to corrupt Riku and has disappointedly accepted Riku can now use Darkness without turning evil.

* [[spoiler:Leo]] in ''[[VideoGame/{{Manhunt}} Manhunt 2]]''. He is a SplitPersonality that is planted by [[spoiler:Dr. Danny Lamb]] in order to pay off his debts. However, it went wrong [[spoiler:when the Pickman Bridge malfunctioned, resulting in Danny being able to see and talk to Leo as if he was a real person]].

* In TheVeryDefinitelyFinalDungeon of ''VideoGame/PlanescapeTorment'', the BigBad traps your mind inside a magic prison, where the personalities of three of your most powerful prior incarnations -- [[AxeCrazy The Paranoid Incarnation]], [[ManipulativeBastard The Practical Incarnation]], and [[TheAtoner The Good Incarnation]] -- [[AmnesiacDissonance splinter from your mind]] and start interacting with you. Reintegrating with your body is only possible through subduing or uniting all four fragments until [[ThereCanBeOnlyOne only one remains]], and the Practical Incarnation isn't particularly well disposed towards [[GrandTheftMe letting]] ''[[GrandTheftMe you]]'' [[GrandTheftMe be the one to do this]].
** In fact, it's implied that being able to resurface and take over your body has been part of his plan all along - a means of surviving beyond death, if his initial plan to defeat the evil had failed.
* In ''VideoGame/BaldursGateII'' the main character ultimately is this, right from the plot premise: [[spoiler:he/she is the child of the now deceased God of Murder, and the recurrent theme is whether he/she will suppress the taint in his/her soul or embrace it.]] He or she then will also learn a new power: [[spoiler:the ability to turn into a powerful beast, the Slayer, at the price of risking his/her own life if the transformation lasts too long (the game will promptly send a warning, with the dialogue menu telling that the main character feels that he/she's losing control)]].

** In ''VideoGame/Persona3'', the members of Strega have Personas that actively try to kill them. To prevent this, they have to take suppressants that have nasty side-effects. [[spoiler:This is because their Personas were forcefully awakened]].
** The Shadows. They're manifestations of humanity's worst flaws and suppressed dark thoughts. It's best shown in ''VideoGame/Persona4'', in which the protagonists each have to face shadow versions of themselves. In this case, suppressing the Enemy Within is the ''worst possible course of action'' since this is what makes them hostile. Accepting their flaws and resolving to deal with them via character development transforms the Shadows into Personas.

* A rather weird example played for laughs is found at the end of ''VideoGame/FalloutNewVegas'' DLC ''Old World Blues'', where you confront [[spoiler:your very own BrainInAJar, who is completely pissed off at you for the various wacky hijinks you've been getting into back in the Mojave at the expense of your body and hygiene. You then have to convince it to cooperate with you and if you want, rejoin your body.]]
** With the appropriate trait you can hit on your own brain in that scene.

* In ''VideoGame/EarthBound'', [[spoiler:Ness's Nightmare is the manifestation of his inner demons within Ness's mind that holds back his full power. After Ness defeats it, his PsychicPowers expand rapidly.]]
* Shiki Tohno of ''VisualNovel/{{Tsukihime}}'' has one in the form of Shiki Nanaya. He is the embodiment of the bloodthirsty, sadistic killer that Shiki fears he could become (or would have become had his life gone a different direction), the one that [[HealingFactor almost killed]] [[LoveInterest Arcueid]].
** He's not the only one in the cast to have them, given the game's general theme of inner demons. [[FriendlyNeighborhoodVampire Arcueid]] has her "[[MonsterProgenitor father]]" the Crimson Moon, who's considering using her as a vehicle for his resurrection. In ''Kagetsu Tohya'', it's revealed that [[HunterOfMonsters Ciel]] has her own evil past self she fears, who [[YourWorstNIghtmare manifests the same way Nanaya does]]; and back in the original work, everyone who gets possessed by [[BigBad Roa]] develops him as an EnemyWithin, including [[spoiler:Shiki, giving him two simultaneous enemies within.]]

* In ''Franchise/TheElderScrolls'' series, Sheogorath, the [[OurGodsAreDifferent Daedric Prince]] of [[MadGod Madness]], has an ArchEnemy in Jygglag, the Daedric Prince of [[ControlFreak Order]] who comes at the end over every Era in an event known as the Greymarch and destroys Sheogorath's realm, the [[EldritchLocation Shivering Isles]]. In the eponymous expansion for ''[[VideoGame/TheElderScrollsIVOblivion Oblivion]]'', it is revealed that [[spoiler:Sheogorath and Jyggalag are one in the same. Sheogorath is the [[SealedInsideAPersonShapedCan Person Shaped Can]] that Jyggalag was sealed within by the other Daedric Princes, who were fearful of his growing power. The main plot of the expansion sees you end this ViciousCycle and free Jyggalag, by [[AGodIsYou taking the mantle of Sheogorath for yourself]]]].

** Sam Vimes contains an UnstoppableRage ("the Beast") which he first kept in check with alcohol and then with [[HeroicWillpower rigid self-control]]. Sometimes he and the Beast have a common goal, though, such as when his [[BerserkButton family is threatened.]]