Basically this is when one of the good guys [[FaceHeelTurn becomes a villain]] not out of choice, but because they were transformed into one one way or another (this makes it a {{supertrope}} to several tropes as outlined below). This can also apply to non-sapient entities, such as computers that [[AIIsACrapshoot go nuts]], {{mon}}s that are TurnedAgainstTheirMasters because someone else is giving them orders, and so on. As long as it was happy with its allegiance and something other than its own whims changed it, it fits.

!!Subtropes include:
* {{Brainwashed}} \\
When someone with a different allegiance takes control of someone's mind, making them do their bidding.
** BeingTorturedMakesYouEvil \\
When brainwashing is accomplished by means of torturing them.
** BrainwashedAndCrazy \\
When the brainwashing is enough to turn the person to violence against their former friends.
** BreakTheCutie \\
When this is accomplished by destroying everything that the person values or holds dear.
** MindManipulation \\
When their allegiance has been forcefully changed by subverting their normal personality.
** MoreThanMindControl\\
When brainwashing together with other factors changes someone's personality or allegiance.
** StockholmSyndrome \\
For when the subject has their personality or allegiance changed by more "organic" means.
* DemonicPossession \\
When something else with a different allegiance hijacks the subject's body.
** ManySpiritsInsideOfOne\\
When it's done by multiple beings at the same time.
** PossessionBurnout\\
The possessed host's body suffers damage and decay that may quickly destroy it from being possessed.
** TransformationOfThePossessed\\
When demonic possession also involves transforming into a demonic appearance.
* HeWhoFightsMonsters \\
When this figuratively happens to the protagonist into becoming one.
** AndThenJohnWasAZombie \\
When this literally happens to the protagonist (assuming their allegiance changes).
** FullCircleRevolution \\
When this applies to LaResistance
** WhosLaughingNow \\
When this applies to a weak character such as the ButtMonkey and TheChewToy who has been picked on endlessly.
** WoobieDestroyerOfWorlds \\
When this applies to TheWoobie
* ReforgedIntoAMinion \\
When the character is turned into one of the villain's minions.
* ResistTheBeast \\
When they try their hardest to stop the inevitable monster from emerging and resist the urge to kill, and warn others around them to stay away lest they face their doom.
* SplitPersonalityTakeover \\
When a different personality starts calling the shots.
** SuperpoweredEvilSide \\
For when the personality is more powerful when it's in control and a little more interested in co-habitation (or at least can't get rid of the "face" personality).
* TragicMonster \\
When this happens either to someone the hero knows and/or cares about, or a backstory is given in such a way that their loss of humanity is to be seen as tragic.
* TranshumanTreachery \\
When the original personality is still there, but their allegiance has changed with their form.
** CyberneticsEatYourSoul \\
When this applies to cybernetic enhancements.
* UnwillingRoboticization \\
When the person is forcibly turned into a robot or other machine.
* TheVirus \\
When the cause of the transformation is being "infected" by the enemy.
** TheCorruption \\
When this gives the infectee evil powers while it mutates him into a monster.
** VampireRefugee \\
When the character fights their transformation and hopes to find a cure before it overtakes them completely.
** ViralTransformation \\
TheVirus causes a physical transformation as well as a [[{{Brainwashed}} mental one]].
* WithGreatPowerComesGreatInsanity \\
Being corrupted by an increase in power.
** PsychoSerum \\
When the serum that gives a person powers also messes him or her up psychologically.
* ZombieInfectee \\
Where this trope could occur at any moment.

!!Aversions and Subversions:
* HeroicWillpower \\
Through sheer willpower, love and loyalty the character maintains their Face status.
* ProHumanTranshuman \\
Despite no longer being human, the character still sympathizes with humans.
* SheepInWolfsClothing \\
When they use their transformation to aid their fellow humans

Please only put examples here that don't fit into any of the above subtropes.


[[folder:Anime and Manga]]
* Happens to [[spoiler:Death the Kid]] in ''Manga/SoulEater'' while [[spoiler:trapped in Eibon's book.]]
** The whole Dark Side of the series is made of [[WithGreatPowerComesGreatInsanity 'insanity']] and its [[SuperPoweredEvilSide effects]].
* [[Manga/YuYuHakusho Urameshi Yusuke]] [[spoiler:pretends]] this (v.1) has happened when [[spoiler:he comes back from the dead the second time. Just to mess with the racist assholes from the Spirit World who'd been sent to vaporize his corpse to make sure he couldn't arise as a superpowered demon. The dub version ends his FaceHeelTurn speech with PSYCH! and in all versions he puts a shoe on his head and sticks his tongue out. Very funny.]]
** Also worked. Freaked out Koenma very badly too, to a WhatHaveIDone level for a couple of seconds over protecting [[spoiler:the corpse.]] Also any viewers who couldn't keep ahead of the plot and predict the crazy man.
* In the Zanpakuto Tales filler arc of ''Manga/{{Bleach}}'', the {{shinigami}}'s {{Living Weapon}}s were given human forms (turning them into something closer to EquippableAlly) and brainwashed with MoreThanMindControl into fighting them. The following arc dealt with [[MonsterOfTheWeek clearing up]] all the "sword beasts" (Zanpakuto who killed their shinigami before being turned back and went rogue).
* In ''Anime/PuellaMagiMadokaMagica'', this happens to [[spoiler:Sayaka and any other MagicalGirl whose [[TransformationTrinket Soul Gems]] don't get purified]].
* In ''Manga/OnePiece'', it's implied this happens to [[spoiler:Bartholomew Kuma as he turns more and more into a robot, to the point that he says that he will technically die soon since his free will shall be taken away from him by the World Government]]. Though only a handful of people are aware that [[spoiler:Kuma]] was ever ''not'' a monster.
* In ''Anime/CuteHighEarthDefenseClubLove,'' the villains are able to turn normal students from the protagonists' school into the MonsterOfTheWeek by corrupting them when they're angry about something.
* In LightNovel/SukaSuka this eventually happens to [[spoiler: the main character after he starts turning into one of the 17 Beasts. While still able to exert some control over his actions, itís a losing battle, and he inevitably becomes the final antagonist of the series]].

* Bruce Banner when he turns into ''Comicbook/TheIncredibleHulk'' (particularly in the original comics where the Hulk started off as a power hungry sociopath who almost considered killing a teenager who knew who he really was).
* In ''Comicbook/{{Amulet}}'', this often happens to Stonekeepers who draw too much on their Amulet's power and become overwhelmed by it in the process, or [[spoiler:fall prey to the manipulations of the Amulet spirit. Particular mention goes to Emily in ''Firelight'', who turns into a giant, flaming, phoenix.]]
* ''{{ComicBook/Revolutionaries}}'': After he dies, [[spoiler:Domitius Major]] is [[BackFromTheDead reanimated by the Talisman]]... as a [[CameBackWrong nightmarish]] WormThatWalks zombie that attacks anything the Talisman tells it to.

[[folder:Films -- Animated]]
* ''Disney/{{Aladdin}}''. When Jafar finally gets his hands on the lamp thus controlling the genie (who has to obey his evil master even though he doesn't want to harm Aladdin).
* In ''WesternAnimation/WereBackADinosaursStory'', when the dinosaurs sacrifice themselves to be part of Dr. Screweye's circus to allow the kids to go free. They get scary looking too...
* Vakama in ''WesternAnimation/{{Bionicle}} 3: Web of Shadows''. While he was shown to be on shaky terms with his team even prior to his mutation into a half-beast, he only turned evil due to Roodaka taking advantage of his bestial impulses.

[[folder:Films -- Live-Action]]
* In ''Film/VanHelsing'', [[spoiler:the titular character]] gets bitten by a werewolf and turns into one. [[spoiler:Although he fights and ends up killing Dracula,]] he is still a monster [[spoiler: and ends up killing his female companion before she can cure him.]]
* In ''Film/ResidentEvilApocalypse'', a hero from the first is turned into Nemesis and sent to fight Alice and the heroes. Eventually she does manage to [[IKnowYouAreInThereSomewhereFight reach him]] and get him to break their control.
* This happen in almost every zombie film, where even the most heroic character could turn into a villain after being bitten by a zombie.
* The clone troops who were forced to turn against the Jedi by an implanted command in ''Film/StarWars: Film/RevengeOfTheSith''.
* In the ''Film/StreetFighter'' movie, this is the goal of Bison's SuperSoldier program, starting by turning Guile's friend Blanka into a green monster. Thankfully, while he was being brainwashed the scientist in charge switched the "Evil" off and turned the "Good" on, allowing for a 40 / 60 % mix of Evil to Good, respectively (more or less, it's been a while).
* In ''Film/IndianaJonesAndTheTempleOfDoom,'' Indy is made into a zombie-slave, though later recovers.

* ''Literature/VampireAcademy'':
** In ''Vampire Academy'', [[spoiler:Natalie turns Strigoi in an attempt to get her father out of jail.]]
** [[spoiler:Dimitri is turned Strigoi against his will]] at the end of ''Shadow Kissed''.
* ''Literature/TheWitchlands'': [[spoiler:Kullen]] turns on his former friends after he Cleaves.

[[folder:Live-Action TV]]
* ''Series/StarTrekTheOriginalSeries'': in the episode "Catspaw", some of the crew are made into zombie-slaves by the witch-alien.
** Previously, it had happened in "Return of the Archons", where Sulu is turned into one of Landru's followers.
* Jean-Luc Picard being turned into Locutus of Borg, in ''Series/StarTrekTheNextGeneration''.
* ''Series/StarTrekVoyager'': in the episode "Equinox", when the captain of the titular ship deletes the Doctor's ethical subroutines, and then has him essentially torture Seven of Nine.
* Subverted, inverted and played straight in the case of Angel from ''Series/BuffyTheVampireSlayer'': he was a jackass who was turned into a monster, was forcibly given back his soul and instead of returning to his jackass original personality, became a good guy, who was then turned back into a monster, and ''back'' into a good guy several times.
** It's the reason he doesn't go by his former human name even when he's a good guy, he's NOT that person anymore. By the time he gets cursed with a soul, he'd been a vampire for about five times longer than he was ever human. He still considers himself a monster, just one capable of feeling guilt for his actions.
* In ''Series/KamenRiderKiva,'' a villain turns Wataru into a scary-looking CGI monster and brings him under his control. He eventually breaks free and uses the monster form to kick his would-be master's ass. We see "Kiva Emperor Flight Style" only once more after that, sadly.
* In ''Franchise/PowerRangers,'' this happens to civilians on an "a few times in some seasons" basis. A person is turned into a MonsterOfTheWeek, and acts like a MonsterOfTheWeek until he is freed. Whether blowing up the monster form with a FinishingMove will free the person or is something that would kill him so the Rangers have to come up with something else varies (though in ''Series/PowerRangersDinoThunder,'' kaboom always works.)
** In ''Series/PowerRangersRPM'', Dillon had cyborg implants that would occasionally try to take over. In the finale, it was revealed that [[spoiler:[[OhCrap this also applies to half the city.]]]]
* ''Series/{{Supernatural}}'':
** [[spoiler:Dean]] is transformed into a demon by [[spoiler:the Mark of Cain]] in the S9 finale, and remained so for four episodes before being cured.
** [[spoiler:Sam]] underwent something similar in S6 due to [[spoiler:Castiel]] accidentally [[CameBackWrong resurrecting him without a soul.]]
* In ''Series/{{Charmed}},'' it became a RunningGag how often each of the girls got transformed into a {{wendigo}}/[[OurVampiresAreDifferent vampire]]/[[OurDemonsAreDifferent demon]]/[[Myth/ClassicalMythology Fury]] and had to be saved by the others. (They also turned into good creatures a lot, too.)

* Happens often in Greek mythology, as looking like a monster is [[BeautyEqualsGoodness apparently synonymous with being evil]]. Scylla and Medusa are both well known examples: women who were cursed for things that weren't their fault and became vicious and destructive as a result.

[[folder:Professional Wrestling]]
* The (indirect){{trope namer}} here would likely be Wrestling/FightingOperaHustle, where the main {{power stable}} the [[Wrestling/NobuhikoTakada Takada Monster Army]], had set out to destroy professional wrestling [[ExactlyWhatItSaysOnTheTin using an army of]] [[WrestlingMonster monsters]] and would {{brainwash|ing}} professional wrestlers, turning them into monsters for its cause.[[/folder]]
[[folder: Theater]]
* ''Tamamo-no-Mae Asahi no Tamoto'': Lady Hatsuhana a.k.a. the titular Tamamo-no-Mae was a decent woman chosen to become the Emperor's concubine before she is possessed by an evil fox spirit for no apparent reason. She then sides with [[TheEvilPrince Prince Usugumo]] to start a coup against the Emperor and take over the country.
* The penultimate boss in ''VideoGame/FreedomPlanet'' is [[spoiler:Milla, who has been [[CorruptTheCutie corrupted]] by Lord Brevon into a monster]]. Lilac or Carol is [[MercyKill forced to kill]] this character [[PlayerPunch in order to advance]]. [[spoiler:Or so it seems. Milla survives, but just barely.]] However, playing as the spoiler character [[SkippableBoss skips this boss battle entirely]], since the character [[DevelopersForesight can't fight themselves]].
* This is what André really is in ''VideoGame/Rayman3HoodlumHavoc'': a [[SentientPhlebotinum red lum]] who got scared, turning it into an evil black lum.
* In ''VideoGame/{{Mother 3}}'', [[spoiler:Lucas's twin brother Claus (who was either seriously injured or dead) is turned into a cyborg and mindless slave by the Pigmask Army.]]
** But we [[IKnowYoureInThereSomewhereFight knew he was in there.]]
* In ''VideoGame/ResidentEvilOutbreak File #2'' the third series Tyrant helps the players at first (since it's been programmed to protect humans and destroy hunters). Then it gets damaged, loses its PowerLimiter, mutates even more and goes crazy.
** Also, if a player dies then they turn into a zombie (or a more powerful RoamingEnemy in one level).
* In ''VideoGame/ResidentEvilCodeVeronica'', Steve is infected with T-Veronica and attacks Claire until ThePowerOfLove returns him to normal (but he dies immediately after).
* Even as far back as the second game, Brad Vickers survives the first ''Resident Evil'' just to show up in ''2'' as an optional zombie encounter. ''3'' shows ''how'' he was zombified.
* In ''VideoGame/CaveStory'', The Doctor feeds [[PsychoSerum red flowers]] [[PlayerPunch to one of your allies]], turning her into a [[TragicMonster frenzied monster]] and [[TearJerker forcing you to kill her]].
** Near the end he also transforms [[spoiler: your friend Sue and his minion Misery into monsters to help him in the fight (Sue gets better, Misery's fate isn't explored but she was apparently able to [[MistreatmentInducedBetrayal send Balrog to help you]] escape from the BonusLevelOfHell).]]
* In ''VideoGame/{{Quake IV}}'', the player character is [[{{Cyborg}} Stroggified]], but his comrades rescue him before his mind control chip can be activated, thus making him a PhlebotinumRebel.
* In ''Zenonia 3: The Midgard Story,'' [[ThatOneBoss Ekinard's]] [[TheVirus toxin]] leaks into the rivers on Maru Island and causes anyone who drinks the water to [[ViralTransformation turn into a monster.]]
* Every single enemy bar the final boss in ''VideoGame/WarioLand 3'' turns out to be a human turned into a monster by the BigBad as revenge for sealing him away.
* ''VideoGame/AoOni'', specifically, version [[spoiler:6. Takeshi even tries to kill himself to avoid this. [[OhCrap It doesn't work.]]]]
* Already teetering on the edge of being utterly mental, {{Tragic Hero}} Siegfried Schtauffen in the original [[VideoGame/SoulCalibur Soul Edge]] is completely consumed by the eponymous Evil Sword when he finally takes possession of it, turning him into the Soul Edge manifest; a rampaging demon-knight known as Nightmare.
* In ''VideoGame/DisneyPrincessEnchantedJourney'', the Bogs are actually captured butterflies that were forced into evil, and return to their natural state once defeated.
* While not on the "Good" side, the scientists turned into purple spider-zombies in ''VideoGame/AzureStrikerGunvolt'' possibly count

[[folder:Western Animation]]
* Blackarachnia from ''WesternAnimation/TransformersAnimated'' actually turned to the side of the Decepticons after mutating into an organic spider.
** Wasp also became a Decepticon after Blackarachnia mutated him into an organic wasp shortly after he vowed revenge on [[KidAppealCharacter Bumblebee]] for (accidentally) sending him to prison, though he'd been trying to take Bee down for some time before. The Decepticon symbol his new form inexplicably has is basically there 'cause the toy has it.
* In ''WesternAnimation/MiraculousLadybug,'' the main villain, Hawk Moth, has the power to [[SuperEmpowering turn normal people]] into the VillainOfTheWeek by preying on them in [[MomentOfWeakness moments of anger or depression]]. One of Ladybug's powers is to purify the akumas that possess them, turning them back to normal.
* Happened in the backstory of ''WesternAnimation/StevenUniverse''. Those monsters the Crystal Gems are always fighting? [[spoiler:Theyíre other Gems. At the end of [[GreatOffscreenWar the Gem War]], Homeworld deployed a FantasticNuke that [[TheCorruption mutated and corrupted]] every Gem caught in itís range. Pearl, Garnet, and Rose were the only Crystal Gems who escaped it and they spent most of their time since capturing their former friends and enemies to stop them from hurting people.]]