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:
[[index]]
* TranshumanTreachery \\
When the original personality is still there, but their allegiance has changed with their form.
** CyberneticsEatYourSoul \\
When this applies to cybernetics
* {{Brainwashed}}
** BrainwashedAndCrazy
** MindControl \\
When their allegiance has been forcefully changed by subverting their normal personality.
** BeingTorturedMakesYouEvil
** BreakTheCutie
** StockholmSyndrome \\
For when the subject has their personality or allegiance changed by more "organic" means.
* 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.
* ZombieInfectee \\
Where this trope could occur at any moment.
* 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.
* 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
** WoobieDestroyerOfWorlds \\
When this applies to TheWoobie
** WhosLaughingNow \\
When this applies to a weak character such as the ButtMonkey and TheChewToy
* 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.
** ReforgedIntoAMinion\\
When the character is turned into one of the villain's {{mook}}s.
* 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.
** TransformationOfThePossessed\\
When demonic possession also involves transforming into a demonic appearance .
** PossessionBurnout\\
The possessed host's body suffers damage and decay that may quickly destroy it from being possessed.
* WithGreatPowerComesGreatInsanity \\
Being corrupted by an increase in power.
** PsychoSerum \\
When the serum that gives a person powers also messes him or her up psychologically.
* 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).
[[/index]]

!!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
----
!!Examples:

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

[[foldercontrol]]

[[folder:Anime and Manga]]
* Happens to [[spoiler:Death the Kid]] in ''SoulEater'' while [[spoiler:trapped in Eibon's book.]]
** The whole Dark Side of the series is made of [[WithGreatPowerComesGreatInsanity 'insanity']] and its [[SuperPoweredEvilSide effects]].
* [[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; when she doesn't clean her [[SoulJar Soul Gem]] she turns into a witch. This will eventually happen to all [[MagicalGirl Magical Girls]] in the series.]]
* 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]].
[[/folder]]

[[folder:Comicbooks]]
* 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).
[[/folder]]

[[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 ''Franchise/{{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]]

[[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 ''StarWars''.
* 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.
[[/folder]]

[[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.]]]]
[[/folder]]

[[folder:Videogames]]
* 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 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.
[[/folder]]

[[folder:Webcomics]]
* In ''WebComic/TheOrderOfTheStick'' this has happened to [[spoiler: Durkon]], who [[spoiler: Malack]] has transformed into a vampire. He currently has no free will (as per D&D rules, which state that newly created vampires are under the control of the vampire who turned them). [[spoiler: Following Malack's unexpected demise, Durkon regained his free will and immediately switched back to his [[TrueCompanions old team]]. His personality is mostly unchanged, although he's a bit more ruthless against his (evil) opponents. It remains to be seen how much his alignment has shifted, or what he'll do to satisfy his thirst, but he still seems friendlier than [[TokenEvilTeammate Belkar]]. Although the latest episode strips subvert this, the vampire is a new personality working for Hel and the original personality is caught in an AndIMustScream.]]
[[/folder]]

[[folder:Western Animation]]
* Blackarachnia from ''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.
[[/folder]]

----