The following tropes have their own pages:
[[index]]
* [[AuthorsSavingThrow/TheBindingOfIsaac Author's Saving Throw]]
* [[UrbanLegendOfZelda/TheBindingOfIsaac Urban Legend Of Zelda]]
* [[WhatCouldHaveBeen/TheBindingOfIsaac What Could Have Been]]
[[/index]]
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* AscendedFanfic: The ''Repentance'' expansion brings over the entirety of the ''[[VideoGame/TheBindingOfIsaacAntibirth Antibirth]]'' GameMod, with some tweaks (notably a new soundtrack which replaces Mudeth's original work[[note]]Though that is still accessible through modding, and Mudeth even made some new tracks for the Afterbirth content[[/note]]) applied to keep it consistent with the rest of the game.
* AscendedFanon:
** The "Muliboom" name for the kamikaze ActionBomb variety of the Mulligan was a FanNickname for a while before being made official.
** Ur-Gurdy is a fictitious boss that spawned an UrbanLegendOfZelda in ''Rebirth''. She was added as "The Witness" in ''Antibirth'' (now renamed Mother), and serves as the DiscOneFinalBoss of ''Repentance''. Funny enough, what started as an UrbanLegendOfZelda ended up becoming official.
*** According to [[https://www.reddit.com/r/bindingofisaac/comments/mhj2my/did_anyone_find_the_secret_secret_super_final/gt52785?utm_source=share&utm_medium=web2x&context=3 this post]], Mother started as a joke by Reddit user [[https://www.reddit.com/user/varkarrus/ varkarrus]].
** In addition to [[spoiler:Mother]], The Visage and Reap Creap are taken directly from [[https://www.reddit.com/r/bindingofisaac/comments/2trm97/versus_screen_mockups/ some of NotYourSagittarius's mock-ups]] prior to [[PromotedFanboy working on Afterbirth]].
** Eye of the Occult and Dream Catcher originated from [[https://www.reddit.com/r/bindingofisaac/comments/2rklib/a_few_item_ideas/ this reddit post]].
** The alternate versions of player characters unlockable in the Repentance DLC were originally known as "B-sides" officially, after a similar mechanic from ''VideoGame/NuclearThrone''. Fans instead dubbed them "Tainted" characters, after the mutated BossInMookClothing versions of certain enemies encountered during the Ascent. Later update logs for the DLC use this name for them.
* AscendedGlitch:
** In the original game, it was possible, with clever usage of the [[LuckManipulationMechanic D6]] and items that reduce its charge rate, to completely exhaust all "pools" the game could pull items from. If another item is produced when the pools are empty, the game would freak out and create an "Undefined" item, with a sprite that scrolls through almost every item sprite in the game and does nothing upon pick-up. Making a second Undefined show up would then crash the game. Come ''Rebirth'', one of the new activated items that can be unlocked is "Undefined", which has a sprite made up of parts of other item sprites and displays the same details the original item did when picked up. Item pools are still exhaustible in ''Rebirth'' (and, as a possible nod to the original glitch, the Undefined item is one of the easiest ways to do it), but it only results in a generic HeartContainer item. In ''Afterbirth+'', an achievement was added called "U Broke It!", which rewards the player for exploiting these tricks and receiving 50 items in a single run, which would normally be impossible.
** The "Buttless Chub" glitch; one of the possible Womb rooms of the original game contained two Larry Jrs. and a Chub. Thanks to the limitations of the game, the Chub spawned without her third segment, leaving her unable to spawn maggots from it. The room still exists in ''Rebirth'', and Chub is still missing her behind despite the new engine being able to handle it.
** The Polaroid was a trinket that let you access TheVeryDefinitelyFinalDungeon in the ''Wrath of the Lamb'' expansion. Originally it was supposed to have a very low drop rate from Mom, but it was accidentally made to drop 100% of the time on release. Rather than patch it out, the devs let it be since the final area was no longer a secret. Come ''Rebirth'', and it's now a passive item so it doesn't take up your trinket slot, and even has a new EvilCounterpart you can take instead.
** One trinket, the Liberty Cap, was glitched so that it sometimes gave the effect of the Compass item (which would reveal all special rooms on a floor) for a single room, rather than act as a random mushroom item. This effect carried over to ''Rebirth''. Strangely enough however, the effect was later removed in ''Repentance'', with the patch notes stating the trinket was "fixed", as if it were still a glitch.
** Isaac's weird hitbox allowed him to walk diagonally in-between spikes and avoid taking damage from them in the original game. This is still possible in ''Rebirth'', but it requires more precision. It was also made an explicate feature of some Devil and Curse rooms, which require you to walk between spikes to pick up items.
** Getting revived by Lazarus's Rags as Tainted Forgotten has Lazarus Risen appear as a pile of bones with Lazarus' head stacked on top of it, but otherwise functioning normally. This was originally a bug during development, but Kilburn found it funny enough to keep it in for the patch release.
** For a short while, using the item {{Suplex|Finisher}} just as Berserk was about to wear off with frame-perfect timing would [[OneHitKill instantly kill]] whatever you Suplexed, including {{Final Boss}}es. While this was patched out, pulling the trick off now will cause a massive explosion that deals tonnes of extra damage.
** For whatever reason, in Patch 17.9 Baby Plum started using the joke death animation caused by the EasterEgg item G FUEL!, where a realistic missile is dropped on her and causes a poorly-cropped explosion, in ''normal runs''. Instead of removing it completely, the following hotfix patch reduced the chance of this animation playing to 0.0001%.
* AscendedMeme:
** The [[{{Vaporware}} Rebirth is Cancelled]] joke that LetsPlay/{{Northernlion}} and his friends started can be one of the messages that you get from fortune tellers. A bit ironic, since NL [[DiscreditedMeme discredited the meme]] [[WhyFandomCantHaveNiceThings because those who couldn't read the sarcasm started giving the devs shit over it]].
** The Bloat is one of the most hated bosses in the game, to the point of having a subreddit dedicated to hating him. In the April's Fool challenge added in ''Afterbirth+'', every single boss is replaced with him, with the only way to get rid of him for a floor is to have your active item act as the Book of Revelations, which still overrides the boss with a Horseman.
** The popularity of Ultra Greed's [[StupidSexyFlanders ample rear]] has become immortalized with an [[https://store.nicalis.com/products/greed-butt-mouse-pad official mousepad from Nicalis]] and later as [[https://twitter.com/edmundmcmillen/status/1407160582801813510 a card]] in the ''Requiem'' expansion of the ''Four Souls'' card game.
** While not within the game itself, the ''Requiem'' expansion of the ''Four Souls'' card game makes reference to both [[https://twitter.com/edmundmcmillen/status/1407159881967149056 "The Isaac of Isaac: Reisaac"]] and the infamous [[https://twitter.com/edmundmcmillen/status/1407161334270095375 "You little F**ker"]] rant, among others.
* BasedOnADream: [[https://steamcommunity.com/app/250900/discussions/0/5017567849303206071/ According to a Discord AMA from Kilburn,]] the Red Key item was originally based on a recurring dream of his where he would find doors that weren't normally there in real life, and wake up before he could open them. Edmund encouraged him to [[spoiler:connect it with the Tainted characters.]]
* ChristmasRushed: Despite the generally high quality of ''Repentance'', it's obvious that it wasn't quite finished in time for release. Almost all of the [[spoiler:Tainted characters]] unlocked at least one functionless item, as did Lilith. Half of the characters had no effect for Birthright, an item that's supposed to do something different for everyone. There are a few items that made [[spoiler:The Beast]] nearly impossible due to wonky physics. [[spoiler:Tainted Lazarus]] was quite literally unfinished according to WordOfGod, and was released in an unplayably weak state. All of this was to avoid the DLC entering DevelopmentHell, since it was already pushed back several ''years'' by the time it came out. All of this was eventually fixed in a massive patch in November of 2021.
* CreatorBacklash:
** Edmund was reportedly pretty disappointed at how [[spoiler:the Lost]] was discovered. Not because it was found quickly, but because people found it by datamining the game, rather than the community figuring out the cryptic hints in-game piece by piece. With the later ''Afterbirth'' expansion, Edmund countered this by ''patching in'' secret material when the community solved the puzzle: it wasn't even present in the game code until it was unlocked!
** [[invoked]] During an interview with Northernlion, Edmund revealed that the only reason ''Repentance'' is such a massive DifficultySpike compared to the base game and ''Afterbirth/Afterbirth+'' is because he wasn't happy with players and streamers breaking the game.
** After the ''VideoGame/{{Mewgenics}}'' page went live on Steam, Edmund did [[https://store.steampowered.com/news/app/686060/view/3677805209186094200 a comparison post]] between Isaac and his next game. In the same post he expressed regrets over the lack of item descriptions in Isaac. While he was fine with it at first, he would later hate the decision as the item pool grew over the years, forcing players to play the game alongside a browser or with the help of mods.
* CreatorsFavorite:
** In [[https://www.youtube.com/watch?v=i2cN4DkEQEE an interview]] with LetsPlay/{{Northernlion}} in 2012, Edmund [=McMillen=] said that Eve was his favourite character.
** [[https://twitter.com/edmundmcmillen/status/896897327956238336 According to this Tweet]], The Forgotten is Edmund's current favourite. Then again, given that it was posted only a few months after the ''fourth'' Booster Pack was released, it's probably him being a TrollingCreator.
* DummiedOut:
** The "Oh No!" / Explosive Diarrhea pill is probably one of the few unintentional examples, disappearing from the game sometime around the ''Wrath of the Lamb'' update. The pill returned not only in a patch to the original game, but also in ''Rebirth''.
** [[http://edmundm.com/post/24457249299/isaac-expansion-expansion As hinted here]], Triachnid was originally cut before the game was released. It wasn't removed ''entirely'', though. Due to a glitch, it originally would replace the Cathedral's TrueFinalBoss. This was eventually corrected, and Triachnid was added in as a normal boss.
** ''Afterbirth'' has an unused boss known as "Skinless Hush", implied to be an alternative form of an existing boss. Both the boss' code and spritesheet are in the game's files, but the spritesheet has only one finished sprite and, if forced to spawn in-game, the boss has absolutely no AI attached to it. Edmund claimed it was a RedHerring for data miners, although Tyrone later posted concept art showing it was at least at one point intended to be used in game. Edmund later confirmed that Skinless Hush [[https://twitter.com/edmundmcmillen/status/1378785476211998722 was not a red herring]].
** There are three unused bosses in ''Repentance'' that were cut for time. Cadavra, a dual-boss of sorts that only exists as two sprites (never finished despite being revealed in a Twitter tease); Raglich, a [[BackFromTheDead posthumous Ragman/Rag Mega]] that can be summoned with the debug console and is mostly functional; and Clutch, a seemingly finished boss with an intro screen and everything that's a KingMook to the new resurrecting skeleton enemies. A more polished Clutch would later be properly added in patch 1.7.8 as a boss in the Ashpit.
* {{Feelies}}: The Nintendo Switch boxed release includes a fully illustrated instruction manual, which parodies the original ''Legend of Zelda'' game manual, as well as some stickers featuring some of the game's characters and enemies.
* FollowTheLeader: The success of ''The Binding of Isaac'' has led to a slew of other {{roguelike}} games imitating its basic gameplay style.[[note]]The qualities of games following the ''Isaac'' formula tend to include the following points: top-down twin-stick shooters that have generally fast-paced gameplay; multiple rooms to explore spread over several randomly-generated floors in a {{dungeon crawler}}-like format; require all enemies in a room to be defeated in order to progress; allow an extremely limited degree of backtracking to pick up passed-over items or explore other paths (you can freely travel between rooms on a floor, but progressing from one floor to the next is a one-way trip); tend to become BulletHell in their later stages if they don't qualify from the very start (especially during [[BossBattle boss fights]]); usually have the player and the enemies follow [[HealthDamageAsymmetry wildly different rules in terms of durability]] (with the player's health typically being defined more-or-less as "number of hits until death", while enemies have larger HP numbers that allow player attacks to deal wildly varying amounts of damage based on their build); feature a wide variety of (randomly-placed) collectible items to adjust the player character's stats and fighting style mid-run; have strong {{macrogame}} elements where additional items, characters, and levels are unlocked through multiple playthroughs and achieving various objectives (with the [[TrueFinalBoss definitive]] FinalBoss usually not being available on the first (or even second, third, or fourth) playthrough); often have some in-universe explanation for why [[DeathIsCheap death is not permanent]] and why death and success alike send the PlayerCharacter back to the start of the journey — sans most if not all of the items and upgrades they acquired during the run (such as [[TimeTravel some]] [[StableTimeLoop form]] of TrickedOutTime, [[WeHaveReserves a hilariously-large supply of interchangeable would-be heroes]], or [[spoiler:the events of the gameplay taking place in a MentalWorld and having [[AllJustADream a loose-at-best connection to in-universe reality]]]]); and often have some late-game twist that turns the initially-presented plot on its head (to varying degrees) which may or may not tie into the reason why death isn't permanent. Some of these points are flexible and can be bent or even disregarded, but others need to be followed nearly to the letter to qualify a game as an "Isaac-like".[[/note]] Derivative games include ''VideoGame/NuclearThrone'', ''VideoGame/EnterTheGungeon'', ''VideoGame/OurDarkerPurpose'', ''VideoGame/{{Monolith}}'', ''VideoGame/SoulKnight'', and ''VideoGame/{{Undermine}}'', among others.[[note]]To go over some of the hand-waves for multiple playthroughs: ''Nuclear Throne'' and ''Enter the Gungeon'' use TrickedOutTime, ''Undermine'' uses WeHaveReserves, and ''Our Darker Purpose'' and ''The Binding of Isaac'' itself use [[spoiler:the "story is not connected to gameplay" MentalWorld approach]].[[/note]] It's gotten to the point where some people, most notably LetsPlay/{{Northernlion}}, have proposed referring to these games as "Isaac-likes"[[note]]specifically, Northernlion said that in the description of [[https://www.youtube.com/watch?v=jvFNt7tQUdM his review for]] ''Our Darker Purpose'', which was released in ''2014''[[/note]].
* HeyItsThatSound: The sound effect for picking up coins in ''Rebirth'' is practically identical to the sound effect for picking up gold in ''VideoGame/{{Spelunky}}''.
* InMemoriam: Creator/SatoruIwata's birthday is December 6th, 1959. On December 6th, 2015, there was a [[http://techraptor.net/content/play-as-satoru-iwata-in-todays-binding-of-issac-afterbirth-daily special Iwata-themed daily challenge]], complete with [[http://techraptor.net/wp-content/uploads/2015/12/Iwata-Issac-1-657x330.jpg custom character graphics.]]
* OrphanedReference:
** Lil Dumpy debuted in ''[[VideoGame/TheBindingOfIsaacAntibirth Antibirth]]'' as a friendly version of the Dumpling enemies that the mod also added. When ''Antibirth'' was officially ported into the main game, Lil Dumpy was added, but the Dumpling enemies were cut, leaving the reference stranded.
** ''Antibirth'' added an item called Jacob's Ladder, a reference to the Biblical story of Jacob seeing a ladder that leads to Heaven. However, ''Afterbirth+'' already had an item called Jacob's Ladder, based on the electric phenomenon. The ''Antibirth'' one was renamed The Stairway when it was ported, but the sprite is still obviously a ladder and it's still unlocked by playing as Jacob & Esau.
* TheOtherDarrin:
** Patty Allen-Scott voiced Mom's lines in the original Flash release, but for ''Rebirth'' and all following expansions her role was recast with Carla Kihlstedt.
** As noted under RealLifeRelative, Isaac's voice actress in ''Rebirth'' Tallulah Rossi was replaced with her brother Viggo for ''Repentance'' as Tallulah's voice had by that time aged out of the child range.
* PromotedFanboy:
** Spider, the mind behind the [=SpiderMod=] program (a modified version of ''Isaac'' that runs in a separate .exe, is more optimized, and adds a number of features, including various cheats and controller support), was hired to do coding work for the remake. ''Afterbirth'' then introduced an item named Spider Mod, giving all enemies individual health bars and showing how much damage your tears do to them, which was one of the functions introduced by [=SpiderMod=].
** [=NotYourSagittarius=], creator of the [[http://www.reddit.com/r/bindingofisaac/comments/2v7aj1/alt_floor_concept_rooms_of_the_basilica/ Basilica]], [[http://www.reddit.com/r/bindingofisaac/comments/2v9mze/alt_floor_concept_rooms_of_topheth/ Topheth]], and [[http://www.reddit.com/r/bindingofisaac/comments/2trm97/versus_screen_mockups/ a few boss concepts]], was [[http://www.reddit.com/r/bindingofisaac/comments/2vhpwt/i_have_good_news_and_bad_news/?sort=top hired]] to work on the ''Afterbirth'' expansion.
** [[https://www.youtube.com/watch?v=zprwTpn2vy4 Missing HUD]], a popular mod for ''Afterbirth'' that gives you a more complete breakdown on your stats (including your devil deal and angel room chances), was made into an official feature in ''Afterbirth+'', called Found HUD in the options.
* RealLifeRelative: The voices of Isaac's family in ''Rebirth'' are done by a real-life family: Carla Kihlstedt as Mom, her husband Matthias Rossi as Dad, and their daughter Tallulah as Isaac who was later replaced by her younger brother Viggo for ''Repentance''.
* SleeperHit: WordOfGod has stated he never expected ''Isaac'' to be so popular. At one point he considered giving it away because he didn't believe anyone would actually want to spend money on something so weird and dark.
* ThrowItIn:
** When an item pool is depleted in ''Rebirth'', all future items from that pool are replaced with Breakfast, a standard HP up. This was apparently a bug, but ended up being the perfect solution to empty item pools [[GameBreakingBug compared to what happened in the Flash version]]. Given that Sacrificial Altar[[labelnote:*]]which ignores pool rules and always spawns Devil items[[/labelnote]] has its own special version of Breakfast with The Pentagram, it's safe to say this is now an intended feature.
** As ''Afterbirth+''’s secret character, you can [[spoiler:spend the last of your health on a Devil Deal as The Soul, then switch to The Forgotten before you die. Although The Soul still dies, the run will continue and you are locked as The Forgotten until you get more soul hearts, at which point The Soul comes back]]. This was caught in playtesting, but kept as it was felt "thematically appropriate".
* TorchTheFranchiseAndRun: ''Rebirth'' does this with the ''Repentence'' DLC. After seemingly every single expansion making Isaac's story worse, ''Repentence'' adds one more TrueFinalBoss. Beat it, and Isaac, free of all his self-hatred and sins, lets go of the mortal coil and ascends to Heaven. There's simply nowhere more for the story to go, and understandably the creators have stated it's the last DLC.
* UncreditedRole: [[https://en.wikipedia.org/wiki/Carla_Kihlstedt Carla Kihlstedt]] as the voice for Mom, as well as the violin composer for a few tracks. [[SubvertedTrope She finally receives a small credit]] as a special guest in ''Repentance'''s [[https://ridiculon.bandcamp.com/album/the-binding-of-isaac-repentance-ost/ OST]].
* WordOfGay: Out of all characters, Edmund has stated that [[https://twitter.com/edmundmcmillen/status/1321662651055796224 The Rainmaker is gay]], making him the only monster thus far to have an explicit sexuality.
* WordOfGod: Some information can be gleaned from [=McMillen's=] Tumblr blog and (now defunct) Formspring, though also be prepared for a fair bit of TeasingCreator.
** For the longest time, it was believed that [[spoiler:Isaac used to have a sister who either left or died]], but WordOfGod has [[https://twitter.com/edmundmcmillen/status/1227014269612351488 confirmed it not to be the case]].
** [[spoiler:Edmund has revealed that Isaac is [[VideoGame/TimeFcuk Steven]], [[http://edmundm.com/post/99746755255/if-everything-in-isaac-is-isaac-himself-does-that thus making TBOI the prequel to TIME FCUK.]]]] Although, this piece of trivia should be taken with a grain of salt, since Edmund is known to be a tease. It most likely has more to do with said games sharing similar themes in the narrative, [[spoiler:with Isaac and Steven choosing to go into their boxes, but only Steven choosing to come out]].
** With Edmund's guest appearance in [[https://www.youtube.com/watch?v=Hh87HkTuZj8&t=53m19s a Roundtable stream]], he confirmed that [[spoiler:the game's narrator, who voices the introduction and occasionally calls out pills/cards, is supposed to be Isaac's dad (or at least [[ThroughTheEyesOfMadness a hallucination of him]])]]. [[spoiler:This would later be more-explicitly confirmed in the final endings of ''The Legend of Bum-bo'' and ''Repentance''.]]
* WorkingTitle: [[https://twitter.com/edmundmcmillen/status/781138197426221056 Two.]] First ''Love Sick'', then ''Mom''. ''The Binding of Isaac'' itself was thought of as a play on ''Franchise/TheLegendOfZelda''.
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