History Main / DangerousForbiddenTechnique

Compare DeadlyUpgrade, DealWithTheDevil, CastFromHitPoints, CastFromLifespan, CastFromExperiencePoints, ExplosiveOverclocking, SacrificialRevivalSpell, and DeathOrGloryAttack. See also GodzillaThreshold and LethalHarmlessPowers.

* ''Fanfic/ThisBites'': Not long after the events of [[Anime/OnePieceFilmStrongWorld Strong World]], Sanji created a culinary example. The Death's Door Lunchbox is a meal that contains enough nutrients to feed an army of normal humans for seven days, which is enough to bring [[BigEater Luffy]] back from the brink of death. He explicitly warns Luffy not to eat it unless he was absolutely starving and needed excess energy, otherwise, the concentration of nutrients could kill him.

** A set of three spells is known as the "Unforgivable Curses"; their use lands an automatic life sentence in Azkaban. This is mostly due to their power; one is essentially a MindControlDevice, one is an AgonyBeam, and one just [[OneHitKill kills]]. It's almost impossible to use them without really, truly wanting the consequences (when they are introduced, the teacher explaining them claims that the entire classroom of fourteen-year-olds he's talking to could pull out their wands and try to use the killing curse on him ''en masse'', and he doubts he would get as much as a nosebleed, and when [[spoiler: Harry]] later attempts to use [[AgonyBeam the Cruciatus Curse]] [[spoiler:when overcome by anger and grief over Sirius Black's death]], the person he uses it on is merely knocked off their feet and explains that using the Cruciatus curse requires actually and genuinely wanting to sadistically torture someone and that righteous anger won't work), meaning that their use proves the user's malice. The only general exception is in dealing with wizarding terrorists on the scale of the Death Eaters, who are essentially outlaws due to how dangerous they are (the only known specific outlying exception involving [[spoiler:assisted suicide]]).

* Creator/MercedesLackey's ''Literature/HeraldsOfValdemar'' has the "Final Strike", in which a mage uses up all his energy in an offensive spell, thus killing himself along with whatever is aimed at. It works, too, in the [[DownerEnding tragic]] ending of the ''Last Herald Mage'' trilogy. Elsewhere in ''Valdemar'', [[spoiler: Vanyel's already massive power results in a Final Strike that is so literally earth-shaking that the geography of northern Valdemar is forever altered]], and in the ''Owl Knight'' trilogy [[spoiler: poor old almost-powerless Wizard Justyn must expend his Final Strike to destroy the single bridge that stands between a marauding barbarian tribe and his fleeing village]].

* ''Literature/ThePendragonAdventure'': It turns out that [[spoiler: all Travelers can warp reality like [[BigBad Saint Dane]] can, but doing so drains the life force of [[{{Heaven}} Solara]]. Saint Dane avoids these drawbacks by relying on a warped version of Solara based on darkness.]]

* Overexert your magic in the world of ''Literature/RiversOfLondon'', and you're liable to give yourself a stroke or cerebral aneurysm. Some specific magical effects also have direct harmful effects, such as the face-warping spell that Punch uses on his possession-victims that [[spoiler: made Lesley's face ''fall off'' when it lapsed]].

* Creator/JinYong, who writes {{Wuxia}} novels, loves this trope, but none are more (in)famous than the Sunflower Manual/Bixie Swordplay Technique (featured in ''The Smiling, Proud Warrior''), which requires its male adherents to [[spoiler: [[CripplingCastration castrate themselves]]]] and slowly makes the practitioner more and more feminized.

* Used ''on'' the main character in ''Series/EliStone''. When Eli needs to see the future with more clarity, he asks his acupuncturist Dr. Chen to use a little-known needle combination known as "The Dark Truth." After doing it once, Eli nearly has a heart attack, which makes Chen swear it off. So Eli winds up going to Chen's DistaffCounterpart and rival. He actually has the procedure done at least three times, which is probably why [[spoiler: his aneurysm gets worse by the end of Season 2]].

** In a rather borderline case, Stannis uses this via Melisandre's [[spoiler: [[LivingShadow shadow assassin]]]] which [[CastFromHitPoints consumes his own energy]]. After using it, Stannis is [[RapidAging noticeably aged]] and Melisandre refuses to do it again because it could kill him.
** Daenerys lets Mirri Maz Duur [[spoiler: cast a dangerous spell to keep Khal Drogo alive]]. It [[GoneHorriblyRight goes horribly right]].
** We learn why exactly warging in humans is strictly forbidden among the wildlings. [[spoiler: There is a reason why Hodor can only speak 'Hodor'...]]

** ''Series/KamenRiderStronger:'' Yes, it's OlderThanTheyThink: long before form changing became the norm, the first Rider to do it has Charge Up, generated by a device implanted Shigeru's body. If he doesn't disperse the extra power in one minute, it will explode, blowing him to very small smithereens. Needless to say, he's never failed to finish the fight and execute his FinishingMove within the time limit. Meanwhile, Tackle has the powerful Ultra Cyclone attack. [[spoiler: Using it while already dying from poison finished her off]].

** [[BewareMyStingerTail Stinger]] of ''Series/UchuSentaiKyuranger'' uses one that involves poisoning himself before he fights his brother in Space 20. [[spoiler: It heightens his fighting abilities, but also endangers him. Worse, his brother turns him against other Kyurangers.]]

* Technical wrestlers generally avoid this as most submissions don't actually put that much pressure on the body. That said, WWE has discouraged many holds which involve the user bridging on their neck, most infmaously Wrestling/BryanDanielson's cattle mutilation, often making mat segments less impressive than those of the other majors.

** At the 2007 Survivor Series, Wrestling/ShawnMichaels and Wrestling/RandyOrton faced each other in a title match where Michaels would get disqualified if he used his finisher, the Sweet Chin Music. To even up the odds, however, Orton's championship would go to Michaels if he got himself intentionally disqualified or counted out. [[spoiler: Orton won the match due to Michaels getting distracted trying to use the move but stopping himself, but then used it on him after the match was over.]]

* In the ''TabletopGame/{{Mystara}}'' setting, Glantrians' use of the Radiance is considered this trope even by the Alphatians, who normally consider even the darkest sorts of magic to be permissible. This isn't because it's dangerous to the wielder (which it is), but because [[spoiler: using the power of the Nucleus of the Spheres threatens to drain all magic from the world, bit by bit]].

* Chapter 2 of ''VideoGame/{{Deltarune}}'' has an alternate story route known as the [[spoiler: [[CorruptTheCutie Weird/Snowgrave Route]]]], where [[spoiler:[[TheCorruptible Noelle]]]] learns a spell called [[spoiler:[[OneHitKill Snowgrave]], which can only be used with the [[CastFromHitPoints Thorn Ring]] equipped.]] If used during the fight where it is first learned, it can only be used once, as [[spoiler:Noelle leaves the party afterwards due to [[MyGodWhatHaveIDone realising what she has just done]].]]

*** Y'Shtola has this with the Flow spell. [[spoiler: It's a dangerous teleportation spell where the user and anyone caught in it is scattered into the aether. It's not until midway through ''Heavensward'' that she's returned from it, but now having ProphetEyes. Thancred ended up NakedOnArrival in the Dravanian Forelands, though now he can no longer use even the basic spells like Teleport or Return, rendering him an utter muggle. Minfillia ran into the Flow spell so she could become one with Hydaleyn. The second time she used it was in ''Shadowbringers'' to escape Vauthry's [[TheDragon general]], Ran'jit, and it was only thanks to Emet-Selch pulling her out of the aether this time that a crisis was averted.]]
*** The Ruby Weapon in ''Shadowbringers'' has "Oversoul". [[spoiler:When activated, Oversoul grants the Ruby Weapon a power boost on the scale of a primal, but at terrible cost: the pilot is absorbed into the Weapon's core and forcibly transformed into a clone of whoever's fighting data is in the Weapon's data banks -- in the case of the Ruby Weapon, Nael van Darnus. And what's worse... the pilot was one of [[HeelFaceTurn Gaius Belsar's]] [[OutlivingOnesOffspring adopted children.]]]] Throughout the ''Sorrow of Werlyt'' storyline, the pilots of the Weapons know that [[spoiler: piloting one win or lose is a suicide mission against the Warrior of Light, especially since the pilots are Gaius Belsar's adopted sons and daughters.]]

* An antagonist example: In ''VideoGame/GoldenSun1'', shapeshifting into dragons seems to be a Dangerous Forbidden Technique of the Mars Clan of Prox, requiring a great deal of energy and completely wiping out the user's abilities afterward. Saturos even warns Felix in the first game that he and Menardi won't be much help after fighting Isaac & Co. as the Fusion Dragon (which they aren't, though [[DoomedByCanon not for the reasons Saturos had expected]]). [[spoiler: When it's {{forced|Transformation}} on the antagonists of ''VideoGame/GoldenSunTheLostAge'', they are left without enough power to warm themselves against the freezing cold of the Northern Reaches, and freeze to death. And then the Wise One forces the transformation on ''[[PlayerPunch your parents]]''...]]

* ''VideoGame/OdinSphere'' has Oswald's shadow form. In gameplay terms, it vastly increases his attack speed and power, at the cost of rapidly eating away as his POW meter (which, if depleted, causes him to become exhausted and unable to move or attack until it refills completely.) In story terms, using it too much will destroy Oswald's soul and turn him into one of the ghostly Revenants haunting Winterhorn Ridge [[spoiler: which eventually does happen in one of the bad ending scenarios, is Oswald is pitted against Onyx.]]
** There's also the Darkova spell, which transforms the user into a massive, powerful Cerberus. The former king of Titania used it once to try and fend off enemy forces and went mad with power, ravaging Titania for seven days until he was finally slain by his son. [[spoiler: Ingway finds out how to use it, and it similarly comes back to bite him as soon as he does.]]

* ''VideoGame/SuikodenIV'' offers us the Rune of Punishment, which drains the user's life every time it's used. The rune itself is sentient and tries to engineer events around it to guarantee it will keep getting used, until eventually its bearer is killed and the rune jumps to a new host, only to begin the cycle anew. Interestingly, the Rune of Punishment governs atonement and forgiveness. [[spoiler: if the player is able to forgive the resident backstabbing friend Snowe throughout the course of the game (and he becomes less and less worthy of forgiveness as time goes on, so it's tough) and recruit all 108 Stars of Destiny, Leknaat appears and says that the rune's time of punishment is at an end, and the time for forgiveness has arrived. In addition to unlocking the most powerful rune attack, which greatly damages enemies and greatly heals allies, the improved Rune of Punishment no longer injuries the user for attacks.]] This, of course, implies that everyone that used the rune before was either a) stupid and greedy, or b) unable to understand the proper implications of forgiveness. Compare the Soul Eater Rune in the first ''VideoGame/{{Suikoden|I}}'' game, which... [[ExactlyWhatItSaysOnTheTin eats souls]].

* In ''Videogame/XenobladeChronicles2,'' there exists a [[BackgroundMagicField Background Magic Field]] called Ether, which also exists in all living creatures. Humans cannot normally access this power, but [[EmpathicWeapon Blades]] can absorb it from the air, then transfer it to their human partners. When the Party meets an enemy who tampers with the ether in the air, preventing their blades from absorbing it, [[spoiler: Vandham]] impales themselves with their own Blade Weapon, using the ether in their own body to power their attacks. [[spoiler: [[HeroicSacrifice He doesn't survive it]]]]

* Jacob from ''Webcomic/DemonFist'' can [[spoiler: teleport]], but doing so damages his body. Using the power to escape with the ''[[CoolShip Hookshot]]'' was what cost him his hand.

** Bloodbending, supposedly the ultimate technique of waterbending, which involves moving around the blood in a living body and making that body do whatever you want it to do. Like the Osmosian example below, it is shown that it is detrimental to the Waterbender's sanity, making them AxCrazy and power-driven. To elaborate, Hama was bent on getting revenge after discovering the technique, Katara almost lost herself while trying to avenge her mother, in which she almost lost control while mercilessly bloodbending the Fire Nation's Guards. Unlike the first two, it's apparently quite easy to learn, at least for a master; it's mostly a matter of being willing to do it once you learn it's possible. Later on in ''WesternAnimation/TheLegendOfKorra'', it's revealed that bloodbending has since been outlawed completely and for good reason, as it turned out a mob boss learned the technique and used it to effortlessly control people whenever he liked. Even more disturbingly, this is also the source of [[spoiler: Amon's ability to remove someone's bending, via combining bloodbending with chi blocking]].

** Since Tikki represents creation and Plagg represents destruction, combining the Ladybug and Chat Noir miraculouses is said to be able to allow the user to make literally ''anything'' happen - hence the BigBad's desire for them. However, Master Fu says that there is an equal and opposite reaction from doing this, so even the most well-intentioned attempt will come at a very high price. [[spoiler: The cost is that the wish will literally overwrite reality with a new one, essentially destroying everything and everyone from the old reality in the process.]]

* On ''WesternAnimation/RegularShow'', Muscle Man enters a bodybuilding contest, but since he doesn't have time to be fully fit, he concentrates on posing technique. As a last resort, he tries to execute a pose called the Shredder, which if done correctly, will "shred" the competition; but if done incorrectly, it causes the poser to ''explode''. [[spoiler: Muscle Man manages to do it perfectly and wins the competition.]]

** In ''WesternAnimation/TheTransformers'', Elita One turned out to have the power to [[spoiler: [[TimeMaster freeze time]]]], but as Alpha Trion had warned her, using it [[CastFromHitPoints drained]] her LifeEnergy to a near-fatal level.

** In ''Anime/TransformersCybertron,'' Vector Prime would [[spoiler: greatly tax himself reversing time by a few minutes, and eventually die by using his time/space powers to get the team through the rift separating Gigantion from the normal universe.]]
* In ''WesternAnimation/{{WITCH}}'', the [[ElementalPowers Guardians]] can choose to transform into living embodiments of their elements, giving them PhysicalGod levels of power -- at the risk of losing their personalities and humanity, and being left open to mental domination from outside forces, such as BigBad Nerissa. [[spoiler: Fortunately, the one time they do this, they manage to come back- barely]].

** Ichigo's fight with Byakuya woke up his [[SuperPoweredEvilSide inner Hollow]] enough for it to want to begin taking over his soul. The more he used his power, the stronger the Hollow became. He eventually learned how to master the hollow, which increased his power in battle. However, it could still take over when he was very close to death, resulting in a truly ghastly outcome for everyone involved. [[spoiler: Only when he learned Final Getsuga Tenshou, which was another example of this trope that destroyed his power after use, was his Hollow fully mastered.]]

* In ''Manga/FullmetalAlchemist'', human transmutation is the sole taboo when it comes to performing alchemy, and rightly so. The law of equivalent exchange is strict -- you must sacrifice equal or greater value, and the value of human life places a horribly high toll on anyone who would dabble in such a practice. While trying to resurrect their mother, Edward lost a leg and Alphonse lost his ''entire body.'' It cost Edward his arm to merely tether Alphonse's soul to a suit of armor in a golem form through a blood seal. And that still wasn't enough for the [[CameBackWrong alchemy to work]]. [[spoiler: Not that it matters as Ed later learns that human transmutation can't bring the dead back to life; what they had done was transfer Al's soul to the collected human body ingredients, with Ed's left leg as payment.]]

* Few would expect this from an anime about the ''circus,'' but the arc of ''Anime/KaleidoStar'' revolved around one of these. There was a Dangerous Forbidden Technique for trapeze and highwire artists, the [[spoiler: Fantastic Maneuvre]], which was ''so'' dangerous and forbidden, Fool refused to even tell Sora what it was because as soon as she heard about it, she would be so obsessed with it that she'd try it even when she wasn't ready, and die. Attempting this Dangerous Forbidden Technique was what killed [[spoiler: Karlos']] best friend and old partner [[spoiler: Aaron Killian]], becoming [[MyGreatestFailure His Greatest Failure]] and turning him into TheAtoner. [[spoiler: And it drives Aaron's son Yuri to seek revenge against Kalos, taking the Stage away from him because he believes he drove his father to basically kill himself.]] Needless to say, by the end of the first part of the series, [[spoiler: Sora and Layla]] perform it successfully. Doing it even once is enough to [[spoiler: cripple an ''already'' injured Layla for life [[CareerEndingInjury and end her circus career forever]].]]

** [[spoiler: Izanagi]] allows users to cast genjutsu on ''themselves'' that can briefly override reality. As a tradeoff, [[spoiler: the eyes used to cast the genjutsu will be permanently blinded.]] The only workaround to this seems to be [[spoiler:possessing the incredible regenerative properties of Hashirama's Senju cells, as Madara was only able to restore the sight of his sacrificed eye by incorporating said cells and awakening the Rinnegan. Also, combining Izanagi and Senju cells extended its duration up to a whole minute per eye]]. Because of this, the Uchiha declared it a Kinjutsu, along with its counterpart,[[spoiler:Izanami, which trapped its target in a StableTimeLoop until tey accepted the outcome they were trying to avoid. Even Edo Tensei-revived shinobi aren't immune to the blinding, as Itachi showed]]

* Everyone seems to have one in ''Manga/ThePrinceOfTennis'': Tezuka Phantom and Zero Shiki Serve, Ryoma's Cyclone Smash, Atobe's Tannhauser Serve, Sanada's Rai, you name it. The most powerful, though, is "Dash Hadoukyuu", [[TheBigGuy Kawamura's]] most powerful shot [[spoiler: until Final Hadoukyuu]]. The original Hadoukyuu already puts a great deal of strain on the arm, while the Dash Hadoukyuu is restricted to once per match. [[spoiler: Naturally, he breaks this rule against Gin Ishida]].

** Dragon Saint Shiryu is warned by his master never to push his fighting technique beyond a certain limit, lest he unleash the "Ultimate Dragon", basically sending himself and his opponent into orbit. Of course, Shiryu ''does'' end up unleashing the "Ultimate Dragon". [[spoiler: But he doesn't die. His opponent Shura has a last-minute change of heart and manages to send him back to Earth with just a very well-timed kick. Seriously.]] That being said, the technique was never used again in the manga. In the [[{{Filler}} next arc]] of the anime, Shiryu attempted it again as a last resort move, but stopped when doing so would destroy a MacGuffin that was needed.

** Using Tsubaki's [[spoiler:Uncanny Sword mode]] takes a toll on Black☆Star's health. Early on, he can only maintain it for a short duration before passing out. At one point, the normally submissive Tsubaki refuses to use it out of fear for her partner's well-being. [[spoiler: Eventually, he finds a way around this, at which point the mode apparently stops having a dangerous effect. Really, all it needed was for Black Star to ''listen'' to others (namely Tsubaki) in order to have a chance of working out the technique, rather than assuming it took only physical strength. Being [[JerkWithAHeartOfGold a decent guy really]], he manages it.]]

* In ''Anime/YukiYunaIsAHero'' there's the Mankai, a SuperMode the Heroes can activate to channel the power of the Shinju itself. However, it comes at the cost of a sacrifice, [[spoiler: namely, one of the Hero's bodily functions, such as the loss of the sense of taste, voice, or ability to use a limb.]]
* In ''Manga/YuYuHakusho'', Hiei's Dragon of the Darkness Flame is an absurdly powerful technique that summons a dragon made of insanely hot hellfire that incinerates pretty much any enemy that comes into direct contact with it, even fire-resistant demons. The catch? If the user isn't sufficiently powerful, the dragon can turn on the caster and devour him. And the first time Hiei used it, he had to offer his right arm as a sacrifice. [[spoiler: Also subverted, as the one moment the attack ''is'' turned on Hiei, he's become powerful enough to control it, not only surviving but also temporarily absorbing it into his body, becoming much, much more powerful.]]

** Naruto's original technique, the Blood Clone Jutsu uses blood to form clones almost as tough as the creator, that return any leftover chakra to the original when the technique ends. But the amount of blood required means that anyone without a healing factor would pass out creating more than one Blood Clone. [[spoiler: Naruto creates a stronger version that has Sakura and Ino heal him and give him plasma pills so he can make a small army of blood clones.]]

** In the sequel, Harry using [[spoiler: his inner Phoenix fragment]] qualifies as this on several levels, mainly because it's insanely volatile. Unless you're incredibly careful, WithGreatPowerComesGreatInsanity, and you end up becoming [[spoiler: the Dark Phoenix]], as he does at the end of ''Forever Red''. Plus, there's the fact that it weakens the already very fragile fabric of reality and risks unleashing [[spoiler: Surtur, the original Dark Phoenix]].

* ''Fanfic/TruthAndConsequences'': Wielding the Ladybug and Black Cat [[TransformationTrinket Miraculous]] [[YinYangBomb together]] is presented as this. In ''theory'', you can become a RealityWarper, capable of making your heart's desire come true. In ''practice'', it requires an incredibly strong will and perfect spiritual balance. Only one person has ever successfully pulled it off, with the consequences of the failures ranging from turning the wielder's hair orange, to transforming them into a potted plant, to [[RetGone Ret Goning]] an entire civilization. Marinette is convinced she can pull it off; everyone else, not so much. [[spoiler: When Gabriel gets his hands on both, not only does he [[CameBackWrong fail to resurrect Emily]] but he himself is seemingly annihilated - taking both Miraculous with him]].

* Although not illegal, a gunsmith in Stephen Hunt's ''The Court Of The Air'' warns Oliver that multi-shot firearms are called "suicide guns" for good reason in their world: bullets are made of glass and explosive tree sap, not metal and gunpowder, so loading more than one round at a time poses a very high risk that the shock wave from your first shot will rupture your second bullet's sap-chambers and make the weapon blow up in your hands. [[spoiler: In the concluding battle, the Court wolftakers reveal that they've licked this problem by cushioning individual cartridges against such shocks and thus, developed glass-bullet ''machine guns'' fed by rubber ammo-belts.]]

** ''Anime/MobileSuitGundamTheWitchFromMercury'': Just piloting a Gundam is this with them banned under the Cathedra Agreement. The Gund format which underlies the technology was meant for human prosthetics, not moving an 18 foot mecha so pilots die quickly from the strain of piloting them. Increasing Permet score boosts performance and the number of drones one can control but also increases the strain even further. The technology is considered cursed and all those involved with it derided as witches. It's implied however that beyond bioethical concerns, [[FantasticRacism the Spacian corporations banning it didn't want Earthians to get a military edge]]. Main character Suletta being able to [[OneHeroHoldTheWeaksauce pilot her Gundam with no visible side effects]] is shocking to everyone

* ''WesternAnimation/Ben10UltimateAlien'': Osmosians are aliens that can absorb matter and create something out of that absorbed material (usually armor). They can also absorb energy, but it can turn them AxCrazy. This was initially used to explain Kevin being less villainous than in ''[[WesternAnimation/Ben10 the original series]] (he used to regularly absorb energy, but eventually stopped), but at one point Kevin has to absorb energy from [[spoiler:Ben's Ultimatrix to stop Aggregor's plans of taking [[RealityWarper a CelestialSapien's power]]]] in a last-ditch effort to prevent the EndOfTheWorldAsWeKnowIt. [[spoiler:They defeated him alright, but afterward Kevin went as far as ruthlessly killing whatever comes his way in a rampage out of insanity.]] He was so insane his wrongdoings can be classified as NightmareFuel.

** On floor excercise, all roll-out somersaults[[note]]save for the simple dive roll, which is so easy it almost never appears in elite floor routines anyway[[/note]] are banned. These skills were banned for women in 1993, which is credited to the paralysis of Russian star gymnast Elena Mukhina while attempting one such skill in 1980; the 2017 code of points also banned these skills for men.

* Twilight mentions in ''Fanfic/GettingBackOnYourHooves'' mentions unicorns have more potential magical power than they can safely use, which operates on the same principle as muscles being capable of far more than they can safely use. Using their maximum power would drastically increase what they can do, but at the risk of damaging or breaking their horn. [[spoiler:At the finale, [[BigBad Checker Monarch]] does this after her EngineeredPublicConfession in an attempted TakingYouWithMe on Trixie and the mane cast. It not only fails but her horn is badly damaged as a result.]]

* Parodied in a ''Fanfic/JusticeSocietyOfJapan'' omake, where [[Anime/DragonBallZ Mr. Satan]] and [[Franchise/StreetFighter Dan Hibiki]] both claim to know one of these, but are both too scared to actually perform it.
* ''Fanfic/TheLegendOfCynderSeries'': Myst's fury, after she takes a potion to help cure her blindness. The potion has a side effect of drastically increasing a dragon's powers the more they drink; Myst needed to drink a whole bucket's worth to purge The General's poison that was blinding her out of her body. It's heavily implied that Myst's fury would be one of the single most powerful attacks in the entire fic series. However, it would have the unfortunate side effect of killing her if she were to release that much power at once. As such, she has never once used it so far.
* In ''Fanfic/{{Origins}}'', a ''Franchise/MassEffect''[=/=]''Franchise/StarWars''[[spoiler:[=/=]''[=Borderlands=]''[=/=]''[=Halo=]'']] MassiveMultiplayerCrossover, Dr. Kevin Filner comes up with one. In order to fight [[spoiler:the Flood]] the heroes need ships that don't have crews. Unfortunately, all the power sources that have been used for various warships so far are either inadequate or run on some form of {{Unobtainium}} like hypermatter. Consequently, he suggests using biotic batteries, like a certain [[HeelFaceTurn villain-turned-at-least-nonthreatening]] -- Sarah. He repeatedly lampshades how much trouble he will be in for suggesting such a thing, and considering these are [[MamaBear Jack's]] students we're talking about, he's probably right.
* Naruto copies the whole scroll of kinjutsu in ''Fanfic/ADropOfPoison''. Some of them are just outright suicidal, but many others are quite useful in the hands of disposable shadow clones. For example, the Evident Hellfire technique allows the user to gather fire chakra in a ball, and just keep gathering it, potentially for weeks on end, so long as they can keep the ball stable. But it doesn't give feedback about how much chakra is inside, and when it bursts, it all goes up in one big bang.
* Subverted in the ''Manga/{{Naruto}}'' story ''Fanfic/{{Vapors}}'', where the Hiraishin should be this given the numerous ways to TeleFrag yourself or worse. In fact, it has several safeguards built in to avert this, although using it [[spoiler: with cranial hemorrhaging]] is still a very bad idea.
* ''Fanfic/RanmaSaotomeChiMaster'' has the Black Dragon Breath Technique, which greatly increases the amount of chi the user has at his or her disposal, and automatically channels it into a dragon-shaped battle aura around the user. The drawback is that it puts a ''lot'' of stress on the user's body. Since the battle aura is so compact, it eventually causes the user's muscles to tear themselves apart. Even worse, the breathing exercises required to maintain the aura starts rupturing the blood vessels in the lungs after a while, eventually causing the user to drown in his or her own blood.

* Parodied in ''WebVideo/DragonBallZAbridged'' when Nail suggests to Piccolo to fuse. Nail tells him that the technique is forbidden even amongst their highest clans... only for Piccolo to ask if they're just going to abuse it. "Maliciously!"
* In ''Fanfic/PokemonResetBloodlines'', [[LimitBreak Z-Moves]] can only be used once per day because they drain both the Pokémon and the trainer of a lot of energy, and it's known that people have ''died'' trying to perform them consecutively within the span of ''hours''. [[spoiler: However, Bloodliners seem to be capable of using them consecutively without fatal risks in the span of minutes with different Pokémon, albeit not without [[PowerStrainBlackout collapsing after doing so]].]]
* In ''Fanfic/{{Transcendence}}'', Ichigo's [[SuperMode bankai]] effectively makes him a juggernaut that can go head-to-head with some of Azeroth's toughest entities. However, his vessel can't withstand the strain his power puts on it, so he can't use his bankai for more than a few minutes without risking it causing [[HeroicRROD heavy damage to his body]]. Even if he doesn't leave it active for long, deactivating it immediately leaves him exhausted. As a result, Ichigo treats bankai as a [[GodzillaThreshold last resort]].
* ''Fanfic/DekiruTheFusionHero'':
** Izuku views Human Fusion as this. While an incredibly powerful technique, it has many downsides. It's CastFromStamina, with even more stamina being drained when using taxing techniques/abilities like One for All, its MentalFusion aspect risks revealing personal information and secrets to all component partners, all injuries endured during the fusion will be split amongst every component partner, and it's ''very'' unpredictable, as the fusions are entirely his or her own person. Unless Izuku has merged with the component partner before, whichever Quirk the resulting fusion has will be completely unknown until he or she figures it out themselves. As a result, it's best used as a last resort. Aizawa actually praises him for this view, because it shows that he won't be a victim of CripplingOverspecialization, which can be fatal for a hero.
** [[spoiler:Multi-person fusions. While they are more powerful (enough to rival ''All Might''), they drain even more stamina than the standard one-person fusion, especially from Izuku. Using a multi-person fusion more than once a day can cause potentially fatal exhaustion to him]].

* In both ''Fanfic/MyHeroSchoolAdventureIsAllWrongAsExpected'' and the [[Manga/MyHeroAcademia source material]], Tokoyami's Quirk, Dark Shadow, becomes incredibly powerful but also impossible to control in absolute darkness. However, this is {{Subverted|Trope}} with Hachiman's [[PowerCopying copied version]]. Thanks to starting at merely 1/108th the power of the original, pitch-black darkness instead just makes Dark Shadow strong enough to be usable even without [[VancianMagic prior Stockpiling]].

* In the ''WesternAnimation/AvatarTheLastAirbender'' fanfic ''[[https://archiveofourown.org/works/3814345 Breath Of Life]]'', Aang reveals that the [[BlowYouAway Air Nomads]] had a bunch of dangerous techniques designed specifically to kill that were invented by rogue Airbenders. The [[MartialPacifist pacifistic]] Air Nomads considered these techniques dangerous because their deadliness went against their way of life and had the knowledge locked up. The reason Aang knew these techniques was because he was forced to learn them when the monks found out he was the Avatar.
* ''[[https://archiveofourown.org/works/25178320/chapters/98212914 That Time I Got Thrown Across Dimensions and Landed On My Big Tiddy Redhead GF Who Is Also A Devil]]'': The second holder of One for All had a Quirk that massively boosted all his physical abilities until his body started tearing itself apart. All For One attempted to use his other Quirks to counter this disadvantage, only to find it boosted his Quirks just as much. He swore not to use it until a century after he stole it, when facing Izuku who was completely overpowering him. In a ShoutOut to ''Manga/{{Naruto}}'', All For One's body starts cooking from the inside as he uses it, eventually blackening and crumbling away to ash.
* Possession is forbidden in the ''VideoGame/NieR'' fanfic ''Fanfic/TowerOfBabel'' for this exact reason. It grants the Shades more power, but there is a high chance of [[WithGreatPowerComesGreatInsanity relapsing]], especially if the host fights back.
* Sanji created a culinary example in ''Fanfic/ThisBites''. The Death's Door Lunchbox is a meal that contains enough nutrients to feed an army of normal humans for seven days, which is enough to bring [[BigEater Luffy]] back from the brink of death. He explicitly warns Luffy not to eat it unless he was absolutely starving and needed excess energy, otherwise, the concentration of nutrients could kill him.

* ''LightNovel/HighSchoolDXD'' gives us Juggernaut Drive, which allows the users of [[EmpathicWeapon Boosted Gear or Divine Dividing]] to completely unseal the Dragon contained within, transforming into a near-unstoppable, monstrous form that goes on a berserker rampage until the user's life is consumed. Both [[TheHero Issei]] and [[TheRival Vali]] have to find a way around this. [[spoiler:Vali has, long before the start of the series -- as a BornWinner, he has a ''stupid'' amount of Devil-power and fuels his JD with that instead of his lifeforce. Issei forgoes the form entirely in favor of creating Cardinal Crimson Promotion, an exhausting but near-equal power boost that relies on his link to Rias, befitting his role as TheChampion.]]

* ''LightNovel/TheRisingOfTheShieldHero'': The Curse Series is quite powerful, although it requires an adequate sacrifice in using its power as its name implies. The most notable attack is Naofumi's [[CastFromHitPoints Blood Sacrifice]] that absolutely '''ensures''' it utterly destroys an enemy, if not bring them to near-death. Unfortunately, as the name of the attack also implies, it expels blood from multiple areas on the user's body in order to act as the "payment" required for using said skill as it acts as the medium to use said skill, and if the user is not treated immediately after, he/she will most likely die and if they survive then he/she will be inflicted with a temporary curse that sharply debuffs his/her stats.

* In ''LightNovel/{{Slayers}}'', Lina's Giga Slave is the mother of all Dangerous Forbidden Techniques. It consists of [[spoiler:summoning a fragment of the [[{{God}} Lord of Nightmares]] into the physical plane]], so not only will she die if it is miscast, but she'll take the entire universe with her. [[spoiler: Naturally, it's miscast, and only a literal DeusExMachina on the part of the Lord of Nightmares saves the day.]] Ragna Blade also consumes magic quickly enough to put the caster's life in danger if they try to maintain it for too long.

* In ''LightNovel/UnlimitedFafnir'', Yuu is able to obtain tremendous power from an ancient dragon calling itself "Yggdrasil" in order to defeat dragons attacking humanity. Unfortunately it requires his memories in order to summon said powers.

* ''Film/{{Ghostbusters|1984}}'': The Proton Packs fire a stream that can snag a ghost like a lasso. But the device is very unstable, and if the proton streams met, it would cause a "total protonic reversal", leading to all life coming to an end simultaneously and every molecule in the user's body exploding at the speed of light. [[spoiler:They're forced to use it to end Gozer's threat at the end of the first movie. Thankfully, all it does is cause a big boom and cover New York in marshmallow, because seemingly all the effects remained on the other side of the portal.]]

* In ''LightNovel/FateZero'', Kiritsugu Emiya can use the "Innate Time Control" magecraft to internally affect time -- speed it up for superhuman reaction time in combat, or slow down his bodily functions to avoid a search-and-destroy system. Bad thing, once Kiritsugu stops using it and his body automatically re-synchs with the rest of the world, he can suffer ''very'' ugly internal injuries. [[spoiler:Only by using Avalon, the scabbard of Saber's sword which grants its bearer regeneration, can Kiritsugu go beyond Double Accel with Time Alter, allowing him to use Triple and Square Accel, and even slow his biological processes down to a third of their normal speed this way]].

* In ''LightNovel/TheIrregularAtMagicHighSchool'', there is a species of alien worm ("parasites") that [[BrainwashedAndCrazy overtakes a host's mind]] to puppet the physical body it leaves behind. People of sufficiently strong will can ''prevent'' a parasite latching onto them in the first place, but there is absolutely no way of reclaiming someone once a parasite has started to burrow. Still, [[spoiler:Minoru]] is frustrated enough by his chronic health conditions that he consciously lets a parasite into his mind [[BattleInTheCenterOfTheMind and fights it for control there]], hoping to use its power to fix himself. This ''works'' (or seems to; [[MoreThanMindControl the nature of parasites]] means that he wouldn't know if he had been overtaken), giving him all the benefits of a parasite connection and none of the downsides. Even so, [[spoiler:Minoru]] must live the rest of his life exiled on a spaceship, because someone willing to take a risk like that- knowing how many people an ''un''controlled parasite would try to kill- cannot be trusted to live among the general population.

* In Anthony Reynolds's ''TabletopGame/Warhammer40000'' novel ''[[Literature/WordBearers Dark Apostle]]'', creative thinking is viewed like this by the Mechanicus. Dangerous enough to be sealed away in a separate brain.

* Robert Jordan's ''Literature/TheWheelOfTime'':

** While it's not really forbidden, lightning redirection (a Firebending technique where you internalize lightning and shoot it in another direction) is extremely dangerous, to the point where even firebending masters are reluctant to teach it, because there's no real way to practice it first. First, you have to ''catch'' the lightning, which is just as dangerous as it sounds. While internalized, the lightning must be carefully controlled so it doesn't travel through vital organs. If the technique isn't performed ''perfectly'', then the user takes the full power of the lightning strike anyway, which is almost always fatal. By the end of the series, only three characters know the technique, and only [[MemeticBadass Iroh]] is skilled enough to avoid the dangers consistently.

* Part of Izuku's CharacterDevelopment in ''Manga/MyHeroAcademia'' is to stop treating [[SuperpowerLottery One For All]] like this trope. Prior to developing the "One For All: Full Cowl", Izuku would pour his power into one extremity, which would deliver devastating blows at the cost of wrecking that extremity. It got to the point where resident healer Recovery Girl put her foot down and refused to treat ''any'' One For All user due to Izuku relying on her healing to mitigate his recklessness. Under "Full Cowl", he instead focuses that power through his entire body and draw out a certain percentage so he isn't wrecking his body. It's still this trope if he goes over a certain percentage, though, but he works to steadily push that percentage upwards.

* ''VideoGame/CopyKitty'': The BonusBoss of the game is a techno-magical abomination that wields the ultimate dark magic spell, something called the "Vestige of the Lost". However, even the boss cannot control the full spell and has to split it into three components which it channels through orbiting altar-like enemies. To defeat the boss, Boki has to [[PowerCopying copy all three component powers]] ([[spoiler:Underworld Fire, Accursed Claw and Profane Edict]]) and and then combine them to unleash the spell's full power. [[RealityBreakingParadox The result is not pretty]], though fortunately this all happens in a simulation, so the worst that happens is the sim crashes and Boki gets knocked out.

** In his battle with [[MadScientist Mayuri]], Uryuu activated [[OneWingedAngel Letzt Stil]], considered the last resort of the Quincy. His [[OldMaster grandfather]] had taught him the technique with the stipulation that [[YouAreNotReady he should never use it]] until he found something he was willing to give up his power to protect. Uryuu chose to use the technique prematurely, found out that ItOnlyWorksOnce, and [[BroughtDownToBadass lost all his spiritual powers]]. His father [[ParentExMachina Ryuuken]] turned out to have an even ''more'' forbidden technique that could restore Uryuu's powers, but it was a very brutal process. [[spoiler:In the Thousand Year Blood War, we learn that there is a way to obtain Letzt Stil powers without the one-time-only drawback, but it is forbidden to traditionalists like the Ishidas as unethical.]]

** Captain Komamura's clan has the Human Transformation Technique, which turns them from anthropomorphic wolves into full humans with a power boost. [[spoiler: It requires literally giving up your human heart, and when the technique runs out, leaves the user as a full non-anthropomorphic wolf]].

* ''Anime/GundamBuildFightersTry'' has "assimilation", a not-very-well-explained phenomenon where a pilot apparently links his mind to his Gunpla, controlling it like his own body and greatly increasing its performance. Downsides? The pilot incurs actual, ''physical'' injuries when the Gunpla is damaged, and on top of that, using this ability is exhausting in the extreme. There is a real risk of receiving permanent or even life-threatening injuries from this, in a sport that's supposed to be completely harmless (at least, to the people playing it).

* In ''Anime/MobileSuitGundamMSIgloo'', the EMS-10 Zudah was a powerful (for its time) Zeonic Mobile Suit with just a [[FlawedPrototype teensy, tiny little flaw]] -- overexerting the engine would cause the Mobile Suit to fly apart. Oh, sure, by revving those engines, you'd get an incredible speed boost, but it's usually not worth losing your life over it.
* The NT-D ("Newtype Destroyer") system in ''Anime/MobileSuitGundamUnicorn''. Built in to the eponymous Gundam, the system activates if it detects a Newtype (or cyber-Newtype) somewhere in the vicinity. It then releases all of the limiters on the Gundam's systems, directly linking all the suit's functions to the pilot's mind. This allows the pilot to gain extreme reflexes, almost mechanical precision, and even [[spoiler:the ability to hijack Psy-Commu weapons, which are generally a Newtype's most dangerous weapons]]. However, this system can override the pilot's own morals with the system's programming to destroy Newtypes and runs a very real risk of burning out the pilot's mind and causing severe brain damage.
* In ''Anime/MobileSuitGundamAGE'', Kio views the AGE-FX's [[SuperMode FX-Burst Mode]] as one, as he has recently made the switch to [[TechnicalPacifist technical pacifism]], and he [[MutuallyExclusivePowerups can't use]] the [[AttackDrone precision weapons]] necessary to guarantee the safety of enemy pilots while using that mode.
* In ''Anime/MobileSuitGundamIronBloodedOrphans'', it's possible to overclock a Gundam's [[BrainComputerInterface Ālaya-Vijñāna System]] to [[SuperMode dramatically increase the suit's performance and reaction time]], but the mental overload from doing so renders the pilot unable to use one or more bodily functions ever again unless they're connected to the machine. [[spoiler:Gundam Vidar]] avoids these drawbacks by [[spoiler:using the Ālaya-Vijñāna System Type-E, which passes the mental strain onto an AI based on [[WetwareCPU Ein Dalton's brain]], which then [[WeaponWieldsYou controls the pilot's body]] to move the machine]].

** Sailor Pluto will die if she uses her power to stop time, and Sailor Saturn will die if she uses her power of destruction. [[spoiler:They're forced to break this taboo in the various series -- in the manga and ''Anime/SailorMoonCrystal'', Pluto breaks her taboo to stop Prince Demando from colliding the past and future Silver Crystals and causing a TimeCrash while in the original anime, she breaks the taboo to allow Uranus and Neptune to enter Mugen Academy and reach Mistress 9 before their helicopter is destroyed; in all three incarnations, Saturn uses her power to stop Pharaoh 90 and allow Sailor Moon to restore everything. In the manga and ''Crystal'', Pluto ends up being reborn as Setsuna Mei'oh and returns to action during the Infinity Arc while in the original anime, her fate is left ambiguous and she sits out ''Super S''; Saturn is reborn as a baby in all three versions and doesn't return until the final arc with Pluto]].

* ''Series/JukenSentaiGekiranger'' features the Ju-Ju-Zenshin-Hen, which allows the user to take on a super-powerful beast-like form. The bad news: transformation is permanent. The worse news: if your technique isn't perfect, your life's gonna ''suuuuuuck.'' A Wolf Fist user winds up transforming into an out-of-control werewolf periodically, but at least he gets restored into his human form. The Fly Fist user (no, really) isn't so lucky, winding up an anthropomorphic fly who is about the size of a mouse. He ends up getting swallowed by an evil Chameleon Fist user and is resurrected along with her after she dies...

* ''Series/MahouSentaiMagiranger'' also has one. Anyone who uses Chronogel's forbidden time spell will die via getting sucked into a wormhole that appears on their chest while it sucks other things like matter and time into it. This curse was deliberately placed on it -- apparently, the use of time magic is so dangerous that ''making sure the universe is screwed if you use it'' is the best way to protect it. Yeeeah.

* [[BewareMyStingerTail Stinger]] of ''Series/UchuSentaiKyuranger'' uses one that involves poisoning himself before he fights his brother in Space 20. [[spoiler: It heightens his fighting abilities, but also endangers him. Worse, his brother turns him against other Kyurangers.]]

* ''Literature/CradleSeries'': The Blackflame Path, a sacred art stolen from dragons, which was used to build an empire. In a world where AsskickingEqualsAuthority and everyone challenges everyone, ''no one'' challenges the Blackflame sacred artists. Not only is it [[CripplingOverspecialization focused on offense to an insane degree]], but the flames are not designed for humans; they slowly eat away at mind and body, resulting in the imperial family slowly dying out without any outside intervention at all. [[spoiler:When Eithan admits that he plans to train Lindon in this path, Cassias starts ''screaming'']].

** The spell Zettaflare shows up, the most powerful BlackMagic spell in the entire Creator/SquareEnix multiverse. Previously in ''VideoGame/BravelyDefault'', it had only ever been used by a FinalBoss being fueled by the power of an evil butterfly. [[BewareTheSillyOnes Donald Duck can cast it on a spur of the moment]] at the cost of knocking himself unconscious for a while. Goofy indicates he's done this before.

* Nova blasts from ''Toys/{{Bionicle}}'', during which the Toa unleash all of their ElementalPowers in a [[FantasticNuke massive explosion]]. Not only does it leave the Toa with no power left, depending on the type of element, it can also easily kill anyone in a close (or even not-so-close) vicinity. As the Toa adhere to a strict ThouShaltNotKill code, the idea of a nova blast is unthinkable, and thus it is the ultimate GodzillaThreshold for a Toa. Only two nova blasts were ever initiated throughout the story's length: the first [[spoiler:by Gali Nuva]] sank an entire island, and was only activated as a last resort to defeat a powerful Makuta after the island's evacuation; while the second [[spoiler:by Jaller]] was being charged up as a desperate attempt to buy one of his teammates enough time to undo the Great Spirit's death and consequent collapse of the universe, even if he would incinerate the rest of his team in the process (fortunately, this situation resolved itself before he actually had to release the blast, and he just barely got it under control before releasing it and consequently [[spoiler:taking all of Metru Nui with him after his team was teleported there]]).

** The spell Zettaflare shows up. Previously in ''VideoGame/BravelyDefault'', it had only ever been used by a final boss being fueled by the power of an evil butterfly. [[spoiler:Donald Duck can cast it on a spur of the moment at the cost of knocking himself unconscious for a while. Goofy indicates he's done this before.]]

*** [[spoiler:In ''Manga/{{Boruto}}'', Kurama reveals another one: Baryon Mode. This grants Naruto immense power, beyond even Six Paths Sage Mode in terms of physical strength and speed, but this comes at the cost of Kurama's life. The only silver lining is that because Kurama's chakra was merely used up rather than foribly removed, it is not fatal to Naruto.]]

*** [[spoiler:In ''Manga/{{Boruto}}'', Kurama reveals another one: Baryon Mode. This grants Naruto immense power, far beyond even Six Paths Sage Mode, but this comes at the cost of Kurama's life. The only silver lining is that because Kurama's chakra was merely used up rather than foribly removed, it is not fatal to Naruto.]]

*** In ''Manga/{{Boruto}}'', [[spoiler:Kurama reveals another one: Baryon Mode. This grants Naruto immense power, far beyond even Six Paths Sage Mode, but this comes at the cost of Kurama's life. The only silver lining is that because Kurama's chakra was merely used up rather than foribly removed, it is not fatal to Naruto.]]

* Parodied in a ''Fanfic/JusticeSocietyOfJapan'' omake, where [[Anime/DragonBallZ Mr. Satan]] and [[StreetFighter Dan Hibiki]] both claim to know one of these, but are both too scared to actually perform it.

** A set of three spells is known as the "Unforgivable Curses"; their use lands an automatic life sentence in Azkaban. This is mostly due to their power; one is essentially a MindControlDevice, one is an AgonyBeam, and one just [[OneHitKill kills]]. It's almost impossible to use them without really, truly wanting the consequences (when they are introduced, the teacher explaining them claims that the entire classroom of fourteen-year-olds he's talking to could pull out their wands and try to use the killing curse on him ''en masse'', and he doubts he would get as much as a nosebleed, and when Harry later attempts to use [[AgonyBeam the Cruciatus Curse]] when overcome by anger and grief over [[spoiler:Sirius Black's death]], the person he uses it on is merely knocked off their feet and explains that using the Cruciatus curse requires actually and genuinely wanting to sadistically torture someone and that righteous anger won't work), meaning that their use proves the user's malice. The only general exception is in dealing with wizarding terrorists on the scale of the Death Eaters, who are essentially outlaws due to how dangerous they are (the only known specific outlying exception involving [[spoiler:assisted suicide]]).

** A set of three spells is known as the "Unforgivable Curses"; their use lands an automatic life sentence in Azkaban. This is mostly due to their power; one is essentially a MindControlDevice, one is an AgonyBeam, and one just [[OneHitKill kills]]. It's almost impossible to use them without really, truly wanting the consequences (when they are introduced, the teacher explaining them claims that the entire classroom of fourteen-year-olds he's talking to could pull out their wands and try to use the killing curse on him ''en masse'', and he doubts he would get as much as a nosebleed), meaning that their use proves the user's malice. The only general exception is in dealing with wizarding terrorists on the scale of the Death Eaters, who are essentially outlaws due to how dangerous they are (the only known specific outlying exception involving [[spoiler:assisted suicide]]).