History VideoGame / TheBindingOfIsaac

** With ''Repentance'' shifting around story elements, the new final villains of the story are [[spoiler:Dogma, Isaac's interpretation of the Christian televangelists that drove his mother to extreme fundamentalism, and The Beast, the Mother of Harlots and the final demon Isaac has to slay. These both precede the reveal that the game's events may be a story being told by Isaac and his dad, and represent the finale of that story]].

** When [[spoiler:Mom's Heart/It Lives]] is defeated, it releases a circle of blood shots. They aren't unavoidable, but a distracted player can be damaged or even killed if not careful.

** Certain characters from other games make cameo appearances as alternate versions of boss fights, like [[VideoGame/TimeFcuk Steven]] for Gemini, [[VideoGame/MeatBoy C.H.A.D.]] for Chub, {{Videogame/Gish}} for Monstro 2, and {{Videogame/Triachnid}} for Daddy Long Legs. The only alternates that aren't cameos are the plot important ones, [[spoiler:It Lives and ???]].

** Isaac:/The Warrior aka, JackOfAllStats: Isaac is not really effective at anything, yet has no real downsides. Any item can be possibly played in his advantage, and a good Isaac player will take what is given to them, and use it effectively. However, after beating [[spoiler:Isaac]] as the secret character, Isaac obtains [[spoiler:The D6]] as a starting item. The item [[spoiler:allows Isaac to transform an item into another one,]] meaning Isaac can have a selection of ''4 items per floor.''
** Cain/The Thief aka, FragileSpeedster: Cain is primarily luck based, with good damage and speed but low HP. His Luck Foot starting items makes him luckier than any other character in the game, and can turn a run around with one good pill or spin of the Gambling Machines. Cain's strength relies on getting lucky, and if it pays out, then Cain can become exceptionally strong.
** Maggy/The Cleric aka, TheMedic[=/=]StoneWall: Maggy has the most starting HP, but is slow and weak. To combat this, Maggy has a starting item (Yum Heart) that can restore a Red Heart every few rooms, keeping her HP high. She can also use this to enter curse rooms without losing health by regaining it using Yum Heart. Maggy's problems can also be taken care of if she gets a Devil item by sacrificing her high HP, meaning that Maggy can trounce the early game.

** Lazarus[=/=]The Paladin aka MasterOfNone: ''Rebirth'' character. Lazarus starts with good health but otherwise terrible stats (including luck), but can revive with great stats but only one health. Pre-death Lazarus can't keep up with the other characters and post-death Lazarus is as fragile as Judas without the massive offense (and leaves you even worse off if it happens late in the run). The only way to make him really effective is to leverage his pre-death health to give his post-death self an advantage; either coast on it until you find a health up item and die on purpose, or go for broke by spending it all on devil deals.

** [[spoiler:The Lost]]/HarderThanHard: ''Rebirth'' character who starts with good items but dies in one hit and can never gain more health, ever. If you discover the complex sequence of events needed to unlock this[[note]]This character's unlock method was greatly simplified in ''Afterbirth'', but it's still [[GuideDangIt pretty obscure]][[/note]], you deserve [[OneHitPointWonder your punishment]]. ''Afterbirth'' makes this somewhat easier by giving him [[spoiler:the Holy Mantle]] as a starter item once you [[spoiler:donate 879 coins to the Greed Machine]], meaning you'll only die if you take two hits in the same room. He can also take Devil Deals for free, so use that to your advantage.
** Lilith[=/=]TheMinionMaster: ''Afterbirth'' character. Lilith is unique in that she starts blindfolded, which means she can't fire tears herself. However, her familiar Incubus copies her tears, which allows Lilith to still benefit from most tear-enhancing items. She starts with the Box of Friends, which creates an additional copy of all your familiars for the room, doubling her DPS (or more if you can find items that let you use the Box multiple times in one room) with Incubus alone, and even more with extra familiars. She also has the Cambion Conception which grants you a demonic familiar permanently after taking damage a significant amount of times.

** [[spoiler:The Forgotten]][=/=]MechanicallyUnusualFighter: Added in the final booster pack to ''Afterbirth+'', is one of the most unusual characters in the game. For starters, they are two characters in one: The Forgotten and the Soul. The Forgotten themselves mainly uses a melee attack via a bone club, which can be charged up to be thrown like a boomerang, and can only gain bone hearts. The Soul functions similarly to the Lost minus being a OneHitPointWonder, but cannot move a certain distance from the Forgotten, and can only have Soul Hearts. Key to playing them effectively is knowing when to use each character, since they have separate health bars and both die when one dies.

** Jacob and Esau[=/=]TagTeam: Added in ''Repentance'', Jacob and Esau are two characters in one, though unlike the Forgotten, both are active at the same time. They both move at the same time and place bombs at the same time, but you can hold a button to only move Jacob in case you need to reposition him. They also do not share items, making choice key to which one you give certain items to, but they also can each carry a spacebar item, effectively giving you two spacebar items available at the same time. Like the forgotten, Jacob and Esau have their own health bars and both die when one dies.

* CrapsaccharineWorld: The game has cutesy ThickLineAnimation, adorable characters, and [[NightmareFuel horrific monsters they have to beat to avoid becoming a red smear on the basement floor.]] Edmund says he did this intentionally to [[http://www.eurogamer.net/articles/2012-06-29-the-binding-of-edmund-mcmillen make the game more palatable]].

*** [[spoiler: Because you're revived on the spot during the Ultra Harbringer/Beast fight instead of being sent to the previous room, Dead Cat becomes an extremely practical item that gives eight more free hits.]]

* DarkIsNotEvil: Dark Bum's item description is "he wants to take your life", which tends to have the player bracing themselves for a crippling PowerupLetdown. His effect is actually literal, because he picks up unused red hearts and converts them into soul hearts. This is almost always an excellent trade (especially if you're playing as [[spoiler:???]], who has absolutely no use for red hearts), making Dark Bum a desirable item

** ''Wrath of the Lamb'' introduces a sort of desperation room, normally accessible only if the player has one heart remaining. These rooms always contain a special item… [[TeleportingKeycardSquad but they're also challenge rooms that spawn bosses when the item is taken]]. Again, [[spoiler:??? and The Lost can always enter these rooms]]. Unfortunately, these rooms were massively {{Nerf}}ed in ''Rebirth'', probably because they were easy to abuse. Items can still show up in these rooms, but it's more likely there will be a plain old chest.

** Guillotine turns your own head into a rotating shield that deals high contact damage. It becomes a bit finicky though since it also moves the area you shoot tears from.

** Dr. Fetus allows you to shoot bombs instead of tears. On one hand, it can hurt you and it takes good timing. On the other hand, you have limitless bombs (enabling you to clear out rocks and get to items and secrets more easily), and enemies and bosses die really easily. This is taken UpToEleven with the Ipecac Syrup in ''Wrath of the Lamb'': it does more damage than bombs and poisons on hit, but its arcing path is very difficult to work with. ''Rebirth'' introduces items like Host Hat and Pyromaniac block explosive damage or cause it to heal Isaac, making bomb attacks always worth using.

*** The Gravity/Brimstone combo in ''Rebirth''. Instead of leaving floating tears that eventually fire unless you release the attack key, it leaves behind timed Brimstone orbs that fire with the full force of a single Brimstone shot. While you have overall less control over your Brimstone, you can charge and fire it much faster since you don't need to wait for the first shot to run out before charging the next.

* DiscOneFinalDungeon: [[spoiler:Chapter 3, where you fight Mom, is only the final part of the first completed run of the game. The following area, [[WombLevel Chapter 4]], was the original VeryDefinitelyFinalDungeon, where you fight Mom's Heart/It Lives. A Halloween patch added Sheol, accessible form a devil room on the Womb 2 or freely after It Lives has been unlocked, where you fight Satan. ''Wrath of the Lamb'' adds the Cathedral as an alternate final floor, with Isaac as its boss. The final patch of ''Wrath of the Lamb'' added the Chest, a follow-up to the Cathedral that ends with ???. ''Rebirth'' would add a dark counterpart to the Chest, the Dark Room, plus a TrueFinalBoss accessible from either floor.]] With ''Afterbirth+'', [[spoiler:a new dungeon was added: the Void. Due to the nature of accessing it, the Void can be reached after nearly any earlier floor to serve as a final level]].

* DungeonBypass: The [[Film/{{Inception}} "We Need to Go Deeper"]] shovel allows you to dig down to the next dungeon level without finding and beating the boss [[ShortCutsMakeLongDelays (and, if you're not judicious about using it, without picking up the powerups you'll eventually need to win the game).]] This can also be used to get to [[spoiler:Sheol]] early.

* EnemiesWithDeath: Death is one of the [[HorsemenOfTheapocalypse Harbingers]]. a collection of bosses. He's the last one to appear, as part of the Chapter 4 floors' boss pool.

** While they usually just feature Isaac getting humiliated in some way, one of Isaac's between-level nightmares shows him suffocating inside of a chest, [[spoiler:foreshadowing the Sheol ending and ???'s possible origins]]. Another nightmare shows Mom getting the message from God, only to cut to [[spoiler:Isacc's ghost/The Lost in the attic, hinting at the fact that most of the game and lore are Isaac's own creation.]]

* GreenAroundTheGills: Picking up Ipecac gives Isaac this appearance, which is fitting since it's a emetic.

** Items that add one extra red heart to your health can be found here and there, often in boss rooms, although only one of them resembles a heart (and an anatomically correct one at that) with most others being food. Some even grant two max HP, and/or restore all your health when you get them.

** The tears' hit box is their shadow, not the tear itself. This is normally a moot point, but bosses like Monstro can be a bit counter-intuitive without knowing this fact. It also makes Ipecac a learning experience, as the arcing tears will detonate despite logic suggesting they would sail over obstacles or enemies. Knowing this makes the Suicide King challenge in ''Rebirth'' somewhat easier.

** Isaac's secret starting item. You need to beat the game's (initial) true final level ten times, which is quite a lengthy and dangerous task. And after that, you need to beat it once again with the resident JokeCharacter, which is nothing short of an extremely painful ordeal. But if you manage to accomplish this mission, your reward will be [[spoiler:[[MundaneMadeAwesome The all-mighty D6 dice]], [[LuckManipulationMechanic an activated item capable of transmuting any collection related item(s) into other random item(s)]]]].
*** As of ''Repentance'', there's an even Infinity +1-er counterpart to the aforementioned item. In Afterbirth+, the [[spoiler:D Infinity]] acted as a random dice item upon being activated. However, in Repentance, it was buffed so that rather than being random, you can ''choose'' which die effect the [[spoiler:D Infinity]] activates, meaning that it allows you to choose from the effects of each of the D1, the D4, the [[spoiler:D6 and its Eternal counterpart]], the D7, the D8, D10, the D12, and the D20. [[spoiler:Unfortunately, the D Infinity doesn't have a Spindown Dice form.]]. Unlike the D100, though, you can CHOOSE to only isolate a particular effect, [[spoiler:which is most useful with the D6 effect]].
*** Speaking of that second spoilered dice item, the [[spoiler:Spindown Dice]], which is unlocked by [[spoiler:Beating Delirium with Tainted Isaac]] is amazing as well. What it does is that [[spoiler:It rerolls all pedestal items in the room into the item that has an internal ID that is one less than its own- assuming you have that item unlocked, of course.]] This may not seem powerful at first, but many common items can be rerolled into possible run-winning items with it. For instance, the Blood Bag, found by donating to a Blood Machine until it explodes, [[spoiler:When rerolled with the Spindown Dice, becomes Brimstone, Lazarus' Rags, usually a not-so-great extra life item, can be rerolled into '''Godhead''' or the somewhat common Bot Fly can be rerolled into the ''Death Certificate'', mentioned below.]]

** The Death Certificate in ''Repentance'' is generally the absolute final item a player can unlock in ''Rebirth'', as it requires getting all Hard Mode completion marks with every character in the game, including [[spoiler:their alternate Tainted forms]]. The unlock is well worth it even if there isn't much to unlock afterwards, as grabbing the Death Certificate [[spoiler:teleports you to a secret area containing one of every unlocked item in the game, letting you pick one (more with exploits) before warping back]].

** [[spoiler:???]], unlocked by beating the true form of Chapter 4's final boss, which itself unlocks after beating the original version ten times. He starts with the same stats as Isaac (not even Cain or Judas's improved offensive stats) except all his hearts are temporary ones like soul hearts or black hearts, which can't be healed like regular hearts can. He can't earn regular hearts, as all health upgrades are converted into soul hearts. He also starts the game with The Poop, which creates a pile of poop. His lack of heart containers prevent many items from doing anything.
** In ''Rebirth'', there's an even larger Joke Character in [[spoiler:The Lost]]. He starts with half a soul heart will constantly be reset to that no matter what he picks up, meaning that you will always die in one hit unless he has extra lives or a form of invulnerability. His only boons are being able to fly you can fly (which Azazel, a much stronger character, can do innately) and being able to take devil deals for free. ''Afterbirth'' buffed him by giving him Spectral Tears and the D4, allowing [[spoiler:The Lost]] to re-roll his build, and makes him start with the Holy Mantle after donating enough coins through Greed Mode, guaranteeing him an extra hit per room unless he reroll it away. He got buffed again in ''Repentance'', by making his Holy Mantle effect intrinsic, meaning it cannot be rerolled away. He also traded the D4 for the Eternal D6, which acts just like [[spoiler:Isaac's D6]] aside from its chance to cause rerolled items to vanish. Unfortunately, devil rooms were changed to only let him take one item.

** ''Repentance's'' [[spoiler:Tainted characters]] all serve as sidegrades to [[spoiler:their originals]], holding significant upsides at the cost of significant downsides, but one that could truly be called this is [[spoiler:Tainted Lost]]. On the plus side, he will never find items that [[spoiler:would otherwise be useless to The Lost]]. On the downside? [[spoiler:He only starts with a Holy Card instead of an instrinsic Holy Mantle, and he lacks the Eternal D6.]] Even worse, his upside is also a double-edged sword [[spoiler:as it also prevents him from finding many items that ''would'' be useful to the Lost. To give a few examples, you can't find any form of extra life items (unless you know how to [[GuideDangIt force them to spawn]])]]. Oh, [[spoiler:and the ''Holy Mantle'' isn't available to Tainted Lost, either, just in case you thought that not starting intrinsically with it wasn't enough of a downside]].

* LethalJokeCharacter: The soul heart mechanic of [[spoiler:???]] lends itself to some amusing exploits that give him an edge. He registers as having no health due to lacking red hearts, so he always has the maximum devil/angel room change, can always enter challenge rooms, and gains permanent use of low-health effects like the Polaroid and Whore of Babylon. He also benefits greatly from items that affect red heart drops or consume them, such as the Mitre and Dark Bum.
** Tainted Forgotten has a very silly premise, where you play primarily as the soul and can only attack by throwing the Forgotten's bones at the enemies which does damage on contact and swings it's bone every second. However the damage of the bones scales to your items you pick up, which means if you got the right set of items it can do absolutely insane damage and make short work of many bosses, especially if they are mostly stationary, including ''[[TrueFinalBoss The Beast]]''.

** The Bible grants flight for a room and doesn't recharge for another six (barring other items to increase charge time), but kills the final boss and the original two {{True Final Boss}}es in one use. However, using it on Satan [[PressXToDie just kills you]], and against every later boss it'll just deal no damage.

** Acquiring four Cubes of Meat requires you to fight and kill all four Harbingers in a single playthrough. Unfortunately, they don't usually all show up on the same run. Acquiring the Book of Revelations early on almost guarantees fighting all four of them, but finding it early enough is another luck mission on itself. It's not even a complete guarantee, as Conquest can replace Death on the Womb/Utero and the Headless Horseman can replace anyone, and they always drop Pony items. In ''Rebirth'', it's even worse, as the Harbingers now have an equal chance of dropping either a Cube or a Ball of Bandages, which also has an achievement for acquiring four. It's overall much easier to use items in later expansions like the Potato Peeler or Box of Friends to get the achievements.

* MadeOfExplodium: Bob's Brain, Bob's Rotten Head and Big Beautiful Fly (BBF) are all reusable bombs. In addition, Ipecac gives Isaac the ability to shoot explosive vomit from his mouth. The reason why any of these things explode is a mystery.

** Guppy's Paw, which trades heart containers for three soul hearts each, isn't so good at the start of the game since red hearts with which to fill your heart containers are easy to come by, unlike soul hearts. However, using it later in the game is beneficial as both are scarce and functionally similar. Spending all of your heart containers will give you three times your max health up to the cap of 48 in the Flash version and 12 in ''Rebirth'', whether your heart containers are empty or not. [[spoiler:Additionally, doing so will give you all benefits of the LethalJokeCharacter.]]

* MindScrew: A whole lot in this game, but one moment stands out. In Ending 6, [[spoiler:Mom's fist flies out of a chest that was spawned by destroying her heart, which was inside her womb, which was inside her corpse in the first place. So Mom was hiding in a chest in her heart in her womb inside ''her'' to begin with.]]
* TheMirrorShowsYourTrueSelf: Zig-Zagged for [[spoiler: Tainted Lazarus. In any room where his reflection is shown, it's instead what his other form looks like.]]

* {{Motif}}: Suicide and self-harm. A lot of the passive items hurt Isaac physically on his sprite, and some items (like Judas' Shadow or Lazarus' Rags) require you die in order to make the most of them. The later bosses factor into this too, with Isaac [[spoiler:basically killing different versions and/or representations of himself]]. The motif makes a lot of sense when the endings possibly reveal that [[spoiler:the whole game is Isaac's DyingDream as he's suffocating in the chest due to perceiving himself as full of sin]].

On June 27, 2018, a Kickstarter page for [[https://foursouls.com/ The Binding of Isaac: Four Souls]], a card game published by [=Studio71=], was launched, and was successfully funded. Another successful Kickstarter page was launched in June 2021 for a massive expansion with the subtitle ''Requiem,'' bringing with it the new content from ''Repentance'' as well as a plethora of crossovers with other franchises, from games like ''VideoGame/CaveStory'' and ''VideoGame/AmongUs'' to even non-video-game works such as ''Film/PGPsychoGoreman'' and ''Film/TheRoom''. The expansion currently has a release window of June 2022. A digital version of ''Four Souls'' is also officially available as a [[https://steamcommunity.com/sharedfiles/filedetails/?id=2501791757 Workshop item]] for ''Tabletop Simulator''.

* BlackoutBasement: The Curse of Darkness results in the entire floor becoming darkened, with most of the visilbility coming from around Isaac's body.
* TheBlank: The angel statue that appears in Angel Rooms as an alternative to deals with the Devil has a blank, featureless face. [[spoiler:This also applies to Uriel and Gabriel, the real angels contained within the statues.]] This may be a reference to the Bible passage Exodus 33:20: ''"But," He said, "you cannot see my face, for man shall not see me and live."''

* AdultFear: The idea of [[spoiler:your son going missing before finding out that he killed himself because he believed he was the cause of his father leaving]] is a very disturbing thought.

** Certain orbitals, such as Cube of Meat and Distant Admiration, can kill bosses relatively quickly if you can stay close enough to deal ContactDamage without taking damage.

* ''Repentance'', another expansion for the remake, released on March 31st of 2021. The expansion is an UpdatedRerelease of the ''[[VideoGame/TheBindingOfIsaacAntibirth Antibirth]]'' GameMod, featuring the alternate path and expanding on it with new secondary floors. It also improves the co-op mode by allowing players to use full-fledged characters rather than babies, and includes a metric boatload of quality-of-life changes to previous content. The release date trailer for this expansion can be found [[https://www.youtube.com/watch?v=gOaWadhNi0A here]].

* SwapFighter: There are a few characters who work like this. [[CantLiveWithoutYou Dying as one character will always kill the other, regardless of their health pool.]]
** The Forgotten is a skeleton that swings a damaging melee [[BadWithTheBone bone club]]. By pressing the drop key, he can switch to a ghost called The Soul, who can freely fly and fires normal tears, but is chained to The Forgotten's inert body. Both characters can only have six hearts at a time instead of the normal twelve.
** [[spoiler:Tainted Lazarus]] switches between himself and [[spoiler:Dead Lazarus]] every time he clears a room or activates his special Flip item. Unlike The Forgotten, the two characters do not share items or stats with each other, but Flip has a passive effect to compensate that lets them collect more items than a normal character.
** The item Esau Jr. lets any character be this. On use, it transforms you into the character Esau Jr., who starts with flight and high base damage as well as a random selection of items equal to your total items when you first used it. Like [[spoiler:Tainted Lazarus]], Esau Jr. doesn't share anything with your original character. Activating again will let you swap back to normal.

* ContractualBossImmunity: [[spoiler:[[TrueFinalBoss Delirium]]]] is is immune to the Chaos Card. which otherwise kills a phase of any other boss in the game when thrown. It isn't immune to [[TakingYouWithMe Plan C]], which destroys everything in a room and kills you three seconds later, but its death animation takes longer than three seconds. Only Lazarus or a character with Lazarus' Rags (both of which respawn the player in the same room) can make use of this strategy, limiting any unlocks to Lazarus alone. ''Repentance'' gives this same immunity to [[spoiler:the Beast, the final superboss.]]

** "Mama Mega!", when used, kills every normal enemy in a room (if an enemy spawns another upon death, you have to kill them yourself), does massive damage to bosses, destroys all forms of rocks (but not Keepers or rocky chests), opens all Secret and Super Secret Rooms, and even opens the paths to Boss Rush and the Blue Womb if you've passed the time limit for either. Best of all, it lasts for the entire floor on which it is used, givinh you an entire "free" floor with nothing standing in your way. The only downside is that it can only be used once before going away unless you picked up a Gold Bomb.

** Downplayed regarding the Ace of X cards. In ''Afterbirth+'', the Ace of X cards turned all pickups in a room into another kind of pickup (hearts, keys, bombs, coins). In ''Repentance'', [[spoiler:they now turn ennemies into their respective pickups as well. Veteran players have learned the effect of these cards in ''Afterbirth+'' and have absolutely no way of figuring out the change if not by sheer luck or accident]].

** The Arcade almost always includes a Blood Bank (which allows you to [[CastFromHitPoints exchange life for money]], with a small chance at other prizes) and a Slot Machine and/or Shell Game beggar (which allows you to exchange money for a random chance at getting pickups, including life). If there are a lot of hearts lying around a floor after you clear it, you can use the Blood Bank to turn the hearts into coins, which can be fed into the Slot Machine/beggar, which will usually give you some hearts, which can be fed into the Blood Bank for coins, and so on. With enough hearts or coins (or the Yum Heart + Nun's Habit, which charges the ability to refill your hearts every time you take damage from the Blood Bank), you can continue this process until one machine or the other blows up, usually yielding a useful item. You're also likely to collect a bunch of Bombs and Keys and probably some Pills and Tarot Cards, and there's no real downside to doing this every chance you get aside from the time investment, which dos limit some of your path options in ''Rebirth''.

* SelfDeprecation: The creators of the original version of the game appear in both the Flash version and ''Rebirth'' as a rare variant of one of the Seven Deadly Sins. [[spoiler:Namely, the the butt-ugly Ultra Pride, due to a feeling of pride and satisfaction at finishing their game.]]

On June 27, 2018, a Kickstarter page for [[https://foursouls.com/ The Binding of Isaac: Four Souls]], a card game published by [=Studio71=], was launched, and was successfully funded. Another successful Kickstarter page was launched in June 2021 for a massive expansion with the subtitle ''Requiem,'' bringing with it the new content from ''Repentance'' as well as a plethora of crossovers with other franchises, from games like ''VideoGame/CaveStory'' and ''VideoGame/AmongUs'' to even non-video-game works such as ''Film/PGPsychoGoreman'' and ''Film/TheRoom''. The expansion currently has a release window of June 2022.

* [[invoked]]DummiedOut: In-universe; the DELETE THIS challenge is made to look like a piece of dummied out content, from the Steam achievement description being a note to the publisher to delete this before the game goes live, it not being properly listed on the menu, and the achievement for beating it having a placeholder name and blank description. The challenge itself looks like a MinusWorld with random music, terrain types, and even glitched-out terrain and a chance to spawn ''actual'' unfinished, dummied out enemies from ''Antibirth''. Also, you start with [[VideoGameRandomizer TMTRAINER]], just to make the whole thing as chaotic as possible.

** Sheol Ending (Halloween update to base game): [[spoiler:Isaac's [[SplitPersonality Split Personalities]], ending with a demonic-looking one (likely an EarlyBirdCameo of Azazel), flash before him. Isaac decides to climb into the toy chest in his room, possibly to kill himself.]]

** Sheol Ending (Halloween update to base game): [[spoiler:Isaac's [[SplitPersonality Split Personalities]], ending with a demonic-looking one, flash before him. Isaac decides to climb into the toy chest in his room, possibly to kill himself.]]

''The Binding of Isaac'' is a [[BlackComedy Black]] [[HorrorComedy Comedy Horror]] {{Roguelike}} Top-Down Shooter made by Creator/{{Edmund McMillen}} and Florian Himsl, released in September 2011.

** Brimstone, a Devil Room item, turns Isaac's attack into a [[ChargedAttack charged]] [[BloodyMurder blood laser]] with infinite range and high continuous damage. It requires paying a couple {{Heart Container}}s to purchase and can be very sluggish to charge depending on your build, along with overriding a lot of other items, so while it can be extremely powerful, it can also reduce your DPS significantly while making it harder to keep enemies away or activate tear effects. Generally, its usefulness varies by expansion/version, as it's an item that frequently gets buffed and nerfed.

* CatScare: The final stage of Greed Mode is named [[spoiler:Ultra Greed]]. At the start of the stage, there's a boss room. Entering the door [[spoiler:reveals a fight against some form of mini-boss (Monstro, a pack of Greeds, Super Greed, or Wrath) that can be mowed down with ease by then. Ultra Greed is in the [[TrickBoss next]] [[YourPrincessIsInAnotherCastle boss room]]]].

* EverythingFades: While no enemies leave behind corpses upon death, they do [[CriticalExistenceFailure explode into]] [[LudicrousGibs blood and bones]] upon death, which is cleaned up upon leaving a room. It's especially noticeable in the Boss Rush if you're quick enough to beat it in order to leave and be able to enter it again, as the room will likely be caked with gore once you're done.

* MadeOGold: The player can find a lot of collectables made of gold, such as hearts, bombs, batteries, and [[UpToEleven even]] [[TheLastOfTheseIsNotLikeTheOthers pills!]]

* AndThereWasMuchRejoicing: [[spoiler:In the epilogue, Isaac rejoices after his AbusiveMom Mom experiences a DeathByIrony]].

[[TvTropesWillRuinYourLife Today I died]]. I was killed by this page on [[Wiki/TVTropes some wiki]]. [-(ugly)-]\\

* AnAdventurerIsYou: Most of the original characters are based on a stock character class: Isaac is a JackOfAllStats, Maggy is a tanky healer, Cain is a lucky thief, and Judas is a GlassCannon attacker. Later characters tend to get more unconventional roles

** ''Wrath of the Lamb'' introduces a sort of desperation room, normally accessible only if the player has one heart remaining. These rooms always contain a special item... [[TeleportingKeycardSquad but they're also challenge rooms that spawn bosses when the item is taken]]. Again, [[spoiler:??? and The Lost can always enter these rooms]]. Unfortunately, these rooms were massively {{Nerf}}ed in ''Rebirth'', probably because they were easy to abuse. Items can still show up in these rooms, but it's more likely there will be a plain old chest.

* FauxHorrific: At the beginning of each level, we see Isaac remembering some awful torture he has been through. In one memory, he is sitting on the toilet and notices there is no toilet paper left -- but his horrified reaction is the same.

** What lowers the chance of getting a Devil Room / Angel Room at the end of the floor changed in ''Repentance''. [[spoiler:All the way up to ''Afterbirth+'', taking any kind of damage to red health would lower the chance. In ''Repentance'', taking damage from Curse Rooms doors or stepping on spikes in a Sacrifice Room no longer reduces that chance. While it is an incredibly useful change, it also makes absolutely no sense for veteran players or for new players. Veteran players have learned for years to not take any kind of damage to red health and new players will certainly take red health damage from other sources than Curse Rooms / Sacrifice Rooms (and it requires them to notice that the chance does not lower, which they may not see if they play without the Extra HUD) before, and will learn to avoid taking red health damage. Moreover, there is nothing in the game that hints at it besides outright taking damage from these sources and realizing that "Oh, I still got my devil room chance".]]
** Downplayed regarding the Ace of X cards. In ''Afterbirth+'', the Ace of X cards turned all pickups in a room into another kind of pickup (hearts, keys, bombs, coins). In ''Repentance'' [[spoiler:they now turn ennemies into their respective pickups as well. Veteran players have learned the effect of these cards in ''Afterbirth+'' and have absolutely no way of figuring out the change if not by sheer luck or accident.]]

* HisNameReallyIsBarkeep: [[spoiler:???]]. It's clear that the character is [[spoiler:a suffocated Isaac]], but for distinction the game always uses [[spoiler:???]] to refer to them.

** ''Repentance's'' [[spoiler:Tainted characters]] all serve as sidegrades to [[spoiler:their originals]], holding significant upsides at the cost of significant downsides, but one that could truly be called this is [[spoiler:Tainted Lost]]. On the plus side, he will never find items that [[spoiler:would otherwise be useless to The Lost.]] On the downside? [[spoiler:He only starts with a Holy Card instead of an instrinsic Holy Mantle, and he lacks the Eternal D6]]. Even worse, his upside is also a double-edged sword [[spoiler:As it also prevents him from finding many items that WOULD be useful to the lost. To give a few examples, you can't find any form of extra life items. Unless you know how to [[GuideDangIt force them to spawn.]]]] Oh, [[spoiler:and the ''Holy Mantle'' isn't available to Tainted Lost, either, just in case you thought that not starting intrinsically with it wasn't enough of a downside]].

** Bosses also contribute to the luck-based nature of the game. Not all of them are equally difficult, and depending on a character you choose and powerups you get, some might be a cake-walk (Larry Jr., Loki) and some an endless source of frustration (Gurdy for a slow-moving character, Monstro II for people who dislike close-range combat, or The Bloat for pretty much everyone). Since not getting hit (which is more difficult if you get a boss you are faring poorly against) is almost a requirement for getting access to the devil room, which often allows one to balance the odds, [[UnstableEquilibrium the difficulty snowballs depending on how much]] the RandomNumberGod loves or hates you.

** The Bible allows you to fly for a single room. While useful, there are items that grant this effect permanently and don't take up your special item slot. It also has the slowest recharge rate possible, so you only get flight once every six rooms (without upgrades). But, if you use it on Mom and Mom's Heart/It Lives, they die instantly, allowing you to bypass two of the hardest bosses in the game and making the NoDamageRun in each section a lot easier. Unfortunately, it doesn't work on other major bosses, making it useless to keep holding onto past that point.

** Void Ending (''Afterbirth+''): [[spoiler:More of a MindScrewdriver than anything else, it explains some things: Isaac's father left his family, and Isaac committed suicide in the chest because he believed he was full of sin. The whole game is a DyingDream, and the last shot is Isaac walking into the distance in the underworld...]]

* PaperThinDisguise: It's heavily implied that [[spoiler:the unlockable characters are just Isaac wearing these.]] Edmund [[http://edmundm.com/post/97038438355/how-old-is-isaac-supposed-to-be-and-are-the-other confirmed it]] in an old blog post.

** A lucky foot usually belonged to a rabbit before becoming lucky--not so in the case of the [[AnArmAndALeg Lucky Foot]] item.

** Prior to their revamp in ''Afterbirth'', sacrifice rooms usually weren't worth the effort - at most, you'll get a chest in exchange for taking a ton of damage.

** The Suicide King special tarot card requires beating one of the more infuriating challenges to unlock and has the splendid effect of dropping 10 items... and killing you.

** Curse of the Tower, unless you have an item that provides bomb immunity or invincibility after being hit. It spawns a handful of troll bombs any time you take damage, dealing lots of damage to anything in the room--but possibly also damaging you again. It will also destroy any blood donation machines or demon beggars you try to use.
** If you manage to defeat one of the hardest bosses in the game with the Keeper, one of the hardest characters in the game, you are rewarded with... him starting with a single penny in future runs. ''Repentance'' thankfully changed this to giving him a penny and his extra HeartContainer.

** ''Rebirth'' introduces the Suicide King, a special card that, upon use, drops ten pick-ups (including golden chests)... and then kills you, [[PowerUpLetDown making it completely useless unless you have extra lives]].

* RandomEvent: The Curses introduced in ''Wrath of the Lamb'', which are randomly activated at the beginning of a level. Curse of Darkness [[InterfaceScrew makes the map useless]], while Curse of the Lost increases the size of a floor from what it would normally be and removes the pattern for Secret Room spawning. Curse of the Labyrinth combines two floors of the same tier -- including two item rooms and [[BossRush two bosses]]. ''Rebirth'' changes the Curses around and adds some new ones.

* {{Roguelike}}: Things like level layouts, monsters, items, and most bosses are randomly generated -- though it's also level dependent (i.e. you will never find a Caves boss in the Basement, unless the boss could appear in both). ''Wrath of the Lamb'' even randomizes what level you'll be sent to by providing alternate stages. For example, in the original version, the first two levels would always be the Basement. After the expansion, each of the first two levels will be either the Basement or the Cellar.
* RummageSaleReject: Given that every passive item picked up includes a cosmetic change to go with it and that these visuals stack graphically as you collect more items, Isaac's appearance by the end of your run is guaranteed to look like this in normal play, when it's not just straight-up BodyHorror.

* SaveScumming: Because the game is randomized, you can instead do this on startup to get preferential powerups instead of garbage (the latter being more likely). This is discouraged in ''Rebirth'' by tracking your current winning/losing streak -- restarting counts as a loss.

** The item "Mom's Eye" makes your tears fire in two directions at once -- in other words, Mom really did have an eye in the back of her head!

** ''Wrath of the Lamb'' added a split path after It Lives, with the Catherdal leading to Isaac, an alternate final boss. By winning enough runs against Isaac and unlocking the Polaroid, Isaac becomes demoted: depending on the version, he is either replaced by ???/Blue Baby[[note]]in the first release of the DLC, Triachnid was mistakenly used in its place[[/note]] or the game progresses onto the Chest, a new final floor for the fight with ???. Satan remains as an alternate final boss.

** The alternate path in ''Repentance'' features a sidequest that unlocks its exclusive final floors and boss, [[spoiler:Mother]]. Defeating it once allows another secret to be reached on any path, whichs leads to a floor containing two more final bosses, [[spoiler:Dogma]] and [[spoiler:The Beast]]. These are considered the definite true final bosses of the game (so long as no more expansions get added), and beating them unlocks one last ending.

** In one of his earliest co-commentaries with Northernlion, Edmund mentioned that he once got a certain combination of power-ups and power-downs that made his firing range too low to reach some monsters standing behind a pit -- and killing all enemies is required to leave a room. This led to a rule change that allows the doors of some rooms to open automatically after enough time has passed.

** If the boss room entrance is facing the top, the trapdoor to the next room will be closed to prevent you from accidentally walking into it. Likewise, if your speed is almost too high to handle, you'll go over the trapdoor instead of into it to keep from going in there because you're too fast.

** The Polaroid and its evil counterpart, the Negative, are now item pickups instead of trinkets. This allows you to pick up the Tick without sacrificing a BrutalBonusLevel run. Effects that reroll a character build, such as the D4 or Missing No. will not effect items like the Polaroid, the Negative, the Key Pieces for [[spoiler:Mega Satan]], or the key items in ''Repentance'' ([[spoiler:the Knife Pieces, Dad's Note, and Dogma]]), so you can never lock yourself out of a path.
** Greed mode's wave spawning can be de-activated by sacrificing half a heart to hit a button surrounded by spikes, but if you're playing as [[spoiler:The Lost]] it's free, since they're a OneHitPointWonder.

** Normally, when you're doing a [[NewGamePlus Victory Lap]], [[NoFairCheating you can't get achievements nor completion marks]]. However, [[spoiler:Delirium]], one the hardest bosses in the entire game. is one of the few exceptions to this rule. This means that you can do a first run to power yourself up, defeat The Lamb, start a Victory Lap and then try to fight [[spoiler:Delirium]], which significantly improves your chances of victory.

** While revival items except for Lazarus' typically revive characters into the previous room, [[spoiler: you're revived on the spot if you die during the Ultra Harbrigners/Beast fight.]]

* ArtShift: The endings added to the game after ''Rebirth'''s launch have a noticeable change in visual style, favoring thicker outlines akin to the original game.
* AsTheGoodBookSays: ''Rebirth'''s title theme is titled "Genesis 22:10", which corresponds to the verse, "Then he reached out his hand and took the knife to slay his son."

** A certain Womb room in the Flash version would spawn two Larry Jrs. and a Chub, but due to engine constraints the Chub would be without her third section and unable to spawn enemies. When the room was ported over to ''Rebirth'', this "buttless Chub" was left in faithfully.

* BabiesMakeEverythingBetter: Implied [[spoiler:in the Final Ending of ''Repentance''. As Isaac ascends to Heaven after defeating The Beast, he sees echoes of his past before him, one of them being his own birth and the faces of his parents filled with joy and ''optimism'' while holding him.]]

* BittersweetEnding: [[spoiler: The true ending of Repentance is this. While it may seem like straight-up happy ending, with the events of the game just being a bedtime story and Isaac’s family never broke apart, WordOfGod confirms that Isaac still died in the chest. However he finally accepts that his family breaking apart wasn’t his fault, defeats the fundamentalist propaganda that drove his mother insane, and finally ascends to heaven.]]

* BonusBoss: ''Afterbirth'' plays with this in regards to [[spoiler:Hush]]. You do have to beat it at least once with every character, but unlike [[spoiler:Mega Satan]], this boss does not end the run. Rather, it is an optional diversion that can net you some extra items at the cost of fighting an extra boss, and once you beat it, you go right to [[spoiler:Sheol or the Cathedral]]. Averted in ''Afterbirth+'', where you have to beat this boss to unlock its FinalBoss.

** Items that grant explosive attacks. Explosions are very powerful and can replace having to preserve bombs, but they can just as easily cause self-damage if the current run doesn't have explosion immunity. They also tend to be saddled with weird traits: Dr. Fetus replaced tears with standard bombs that are subject to physics and need to be timed right against defensive enemies, Ipecac shots fire at an odd lobbing arc, and Bob's Brain, Fire Mind, and similar items require being careful about their positions or their random activations.
** Due to the game's randomization and the built-in "special items" pool (which gets less common as more items from it are received or seen from it in a run), certain combinations of game-breaking items are theoretically absurd but very unlikely to come up in the same playthrough.
** When Isaac is down to his last half-heart, the Scapular item grants a last-gasp reprieve of one soul heart. When combined with the Cursed Skull trinket (which teleports the player to a safe room when they have half a heart left), the player is functionally invincible at the cost of having to re-attempt the room he died in from the start. Good luck fighting bosses under no-hit conditions.

** [[VideoGame/PokemonRedAndBlue Missing No.]] completely randomized your items and stats at the start of every floor. The chance of this turning a poor run into a game-breaking one is much less common than getting stuck with something unworkable.
** The Mega Blast. When you use it, you release a massive blood beam for 15 seconds, letting you utterly decimate everything for that period of time, and it even persists between rooms. It takes 12 charges to use (normal active items require 6 at most) and it's very difficult to unlock ([[spoiler:beat Mega Satan with every character, including the hidden ones]]). It also pushes you in the opposite direction of where you fire it, which can be counter-intuitive and can shoot you right into spikes or enemies. With enough charge-reducing items or skilled use, it can still be worth carrying for a while run.

** Plan C, a one-time-use activated item that deals 9,999,999 to every enemy in the room, and then kills you three seconds later. This will let you effortlessly beat any boss, but must obviously be used with extra lives. The one exception is [[spoiler:Delirium]], as its death animation takes longer than three seconds, and the game does not count it as dead until it is over, so you'll have to start the fight over again. The only counter to this is Lazarus's Rags, the only resurrection item that respawns you on the spot instead of outside the room, but also means Lazarus will get the post-it credit for it. ''Repentance'' adds Inner Child, which lets any character respawn on the spot.
** As of ''Afterbirth+'', Brimstone and Ipecac combined. The massive damage increase from Ipecac now works in conjunction with Brimstone's massive damage multiplier, resulting in a Brimstone beam which does 565.5 damage in total (with Isaac's base stats), enough to one-shot some of the Womb bosses. However, the tear delay increases from both Brimstone and Ipecac are combined so the Brimstone beam takes a huge length of time to charge. In addition, Ipecac shots are fired when charging the beam. To charge it fully you have to continuously fire Ipecac which will literally blow up in your face. To avoid damage you're probably going to be forced to tap-shoot Ipecac, which also has a much lower rate of fire due to the synergy, and not get any benefits of Brimstone.

** [[{{Denouement}} While needing to be pieced together from multiple different endings]], the endings added by the expansions [[spoiler: (as well as the an ending in ''The Legend of Bum-bo'' and [[https://twitter.com/edmundmcmillen/status/1385112195772473348 Word of God]])]] make it clear. [[spoiler:[[TheHeroDies Isaac suffocates to death]] inside the chest trying to hide from his abusive mother while [[DyingDream hallucinating the entire game]] due to asphyxiation fuelled delirium and his father encouraging him to use his imagination as a coping mechanism. His mother believes him to be missing and after an indeterminate period of time she finds his remains and mourns his death. Isaac's fate in the afterlife, however, is ultimately unknown and is left up to interpretation]].

** The endings added by the expansions make it clear [[spoiler:Isaac thought himself to really be corrupt by sin and was driven to suicide]]. The 20th ending in ''Afterbirth+'' puts everything [[MindScrewdriver more or less into perspective]]. [[spoiler:Isaac starves and suffocates to death inside the chest while hallucinating the entire game. His mother believes him to be missing and only finds out the truth when there are only bones left inside the chest. We then see Isaac walking into the underworld, followed by a demonic shadow.]] While ambiguous, [[spoiler:the final ending of ''Repentance'' twists things into a more optimistic conclusion]].
** The Greed mode's ending features Isaac [[spoiler:getting trapped by a cave-in and becoming Greed after hanging himself. In the Greedier ending, his head then comes off and a fountain of bugs gush out of it. [[GainaxEnding How this fits with the other endings is anyone's guess]]]].