Becoming the Costume

"Witch: Taint of mutant, scum of pond, thou shall become the guise thou donned!

Crowd: (confused) Speak English!

Witch: (exasperated) I'm turning you into your costumes.

Homer: Well, are ya gonna talk about it or are you gonna do it? (zapped)"

- The Simpsons

It's All Hallows Eve and the kids are trick-or-treating and everything is normal, but something magical is afoot, and turns people into whatever costume they are wearing. If you're dressed up as a werewolf, for example, you'll become an actual werewolf.

You'd be surprised just how often this happens in fiction.

Not to be confused with Becoming the Mask or Evil Costume Switch. Can overlap with The Mind Is a Plaything of The Body, which is bad news. Related to Evil Mask.

May be the subject of a Halloween Episode.

Anime and Manga

 * In Berserk leader of pagan-cult dressed in goat's head with figure of snake as fake penis gets transformed into an Apostle Spawn, which turns him into real goat-headed, satyr-like monster with snake penis.

Comic Books

 * At least one character in Therefore, Repent! transforms thus.
 * Happens at a revived pagan festival in Hellblazer, to the detriment of everybody (new father in baby costume to infant: "You're taking all of mummy's attention and it's! not! fair!" *splat*; a dentist(?) with a rictus grin mask wants EVERYONE TO SMILE!). This isn't transformation, but rather a crazy-making signal coming from the nearby underground nuclear base. John and his friend discover that loud punk rock cancels the effect
 * This is a major plot point in the indie comic series Underburbs.

Film

 * The Disney Channel Original Movie Halloweentown II had this, with the villains giving the heroine's mother dress up in a really monstrous costume so that she'd become a really scary monster.
 * This happens in the 2003 movie Hallow's End, in which the characters become their costumes when they die.
 * The TV special Witch's Night Out (1978) has a witch turn two kids and their babysitter into the monsters they're costumed as. When the adults find out and turn on her, she explains that the point of Halloween is to try out different personas and imagine your potential. The adults happily start calling out requests for one-night transformations themselves.
 * Angela Franklin of Night of the Demons fame was prone to doing this to her victims; in the third film alone she turns characters into their demon, Catgirl, and snake-armed gorgon costumes.

Literature

 * The Goosebumps book The Haunted Mask (also adapted for TV) had this.
 * Flossie Teacakes' magical coat may be a benign version of this- a particularly fabulous coat that she finds in her older sister's room turns out to magically transform dowdy ten-year old Flossie (when she plays at dressing as her sister) into the young woman that she dreams of being.
 * In one of the Doctor Who Expanded Universe books, Forever Autumn, this happens because of some strange occult-sciency aliens wanting to harvest fear from everyone so they can leave Earth. Yes, Clarke's Third Law is in effect here.

Live Action TV
"Abed: "I am Batman.""
 * Buffy the Vampire Slayer Season 2, Episode 6: "Halloween". This episode is one of the fandom's favorite launch points for crossover Fan Fics, to the point where "YAHF" (Yet Another Halloween Fic) has entered the fandom vocabulary.
 * Lampshaded in the Season 4 episode "Fear Itself" when the gang attend a college Halloween party and all dress up as someone or something that would actually be useful should history repeat itself.
 * Not EVERYONE. Xander and Oz were, though. James Bond and... God.
 * And Willow was Joan Of Arc. Even Anya, who wasn't around for the earlier episode, dressed as something she thought was terrifying. So really, the only one who didn't learn was Buffy, who dressed as Red Riding Hood.
 * The same trick was pulled on a convention in the later Angel comics, turning a bunch of nerds into superheroes and villains, and turning Spike into Angel. Or at least, Angel as Spike sees him.
 * The Twilight Zone had "The Masks", where a dying millionaire forces his worthless heirs to wear masks caricaturing their worst personality traits - if they take them off before midnight, they get cut out of his will. At midnight, he dies, and they shed the masks...
 * Abed in the Community episode "Introduction to Statistics",

Tabletop Games

 * Magic the Gathering: In the Ravnica tie-in novel, Agrus Kos and his angel partner, Feather, arrest a troupe of actors after a faulty "Performance Enchantment" causes the man playing a bloodthirsty Gruul war chieftain to get a little too in-character. Fortunately, nobody is injured.
 * In Monte Cook's World of Darkness, all of the costumes at the Guthrie Theater in Minneapolis work this way. Once someone puts on a costume, they become that character--even when the costume is removed. The effect is apparently irreversible. The theater is popular with suicidal people; by putting on a costume, they can cease to exist and instead become someone else entirely.

Video Games

 * This is the entire plot of a downloadable game called "The Theater." It can be downloaded here: http://www.tanuke.com/?p=1804
 * The Deku, Goron, Zora, and Fierce Deity masks in Majora's Mask are much more than facial accessories...
 * This is a gameplay mechanic in Costume Quest, a Playstation Network and Xbox Live Arcade title from Double Fine.
 * Kid Chameleon on the Sega Genesis as the main gameplay mechanic.
 * Played with in a skit in the PS 3 version of Tales of Vesperia, where the party unintentionally start acting like the Tales characters they're dressed as. The same costumes also let them use an arte that was used by the character it belonged to.
 * This is also the primary gameplay mechanic of the Narikiri Dungeon series, where donning the costumes of different classes, monsters, or Tales characters turns you into them and allows the use of the appropriate abilities.

Web Original

 * The Xanadu storyverse is all about this. At a costume convention with ten thousand in attendance, costumes become real and those wearing them are changed. Mentally the transformation varies from person to person, from "Suddenly my costume is much more accurate" to "Where am I, what is this planet?" Stories tend to be either furry or Welcome to The Real World.
 * Overlaps with Dragged Into Drag on sites like Big closet top shelf all the time.
 * Discord inflicted this upon Ink City when he crashed their Halloween party. This included several residents temporarily transforming into each other thanks to their costume choices, like Rigby becoming Trevor, and Yakko becoming his own player.
 * In general, this is a favorite Halloween event for Journal Roleplay games.

Western Animation

 * The Fairly Odd Parents Season 2, Episode 13: "Scary Godparents".
 * In The Simpsons "Treehouse of Horror XVI" third segment, "I've Grown a Costume on Your Face," a witch curses the town by making them become their costumes.
 * On the Darkwing Duck episode "Slaves to Fashion", Tuskernini, the villainous walrus filmmaker, creates a gas that causes people to take on the personality of whatever costumed character they dressed up as. Hilarity Ensues. The gas doesn't actually affect our hero, though, because he came to the party as himself.
 * The "Curse of Necklace" episode of Martin Mystery.
 * Happened in Danny Phantom special, "Reality Trip" where a Reality Gem transformed a trio of nerds at a comic convention. Unfortunately for Danny, they were dressed as super villains.
 * The Halloween TV special (originally CBC) Witch's Night Out.
 * The Creepschool episode "Tricks That Treat" does this.