Becoming the Costume

L - R: Emily, Wilfred, Wilfred Jr., and Paula after receiving some karmic justice.

Witch: Taint of newt and scum of pond, thou shalt become the guise thou donned!
Carl: Speak English, moron!
Witch: I'm turning you into your costumes!

— The Simpsons, "Treehouse of Horror XVI"

It's All Hallows' Eve (usually) 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 (which is about a pretense turning sincere) or Evil Costume Switch. Can overlap with The Mind Is a Plaything of the Body, which is bad news. Related to Evil Mask. Hilarity might ensue if overlapped with For Halloween, I Am Going as Myself. Can lead to a Monster Mash if there is a large assortment of costumes.

Compare Clothes Make the Superman and Clothes Make the Maniac.

May be the subject of a Halloween Episode, and Special Occasions Are Magic if it happens because it's Halloween.

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Examples:

Anime & Manga

• Berserk: The leader of a cult dressed in a goat's head and used a snake as a fake penis. He gets transformed into an Apostle Spawn, which turns him into a real goat-headed, satyr-like monster with a snake for a penis. Before his transformation he was about to perform a sexual ritual with Casca, who was believed to be a goddess by the cult. After transformation the only thing he can think of is raping her. Luckily for her, he runs into Guts.

Comic Books

• Doctor Who Magazine has one issue where the backstory involves all the nobles of a planet being turned into the animals that they dressed up as in celebration of surviving a plague that killed ten thousand peasants.
• Hellblazer: Happens at a revived pagan festival, 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 a little too late: an ex-fighter pilot is "cured" just seconds after he releases a bomb, destroying the town and everyone in it (except John, of course).
• Silverblade: The longer Jonathan remains in the form of one of his movie characters, the more he starts to believe he is that character. While in the form of Dracula, he gets staked through the heart and 'dies' (because that would have killed Dracula in the film). When Milestone finds his skeleton and removes the stake, that 'revives' Dracula (again because it would have in the film). However, because of the time he has spent 'dead', Jonathan now believes he truly is Dracula.
• Therefore, Repent!: At least one character transforms thus when Raven takes off her bird mask to reveal that she now has the head of an actual raven.
• The Underburbs: This is part of the premise — Winifred, the main villain, wants to conquer the world using a scroll that turns people into monsters based on what they're dressed as — and Played for Laughs. The main character, Angie, and her mother turn into witches, and her brother into a monster. None of this affects life in the town very much.
• The Vision and the Scarlet Witch: Three trick-or-treaters are transformed into a ghost, a goblin and a Jack o'Lantern-headed monster in issue #1 after the demon Samhein escapes from the Druid Tome.

Fan Works

• The focus of the Yet Another Halloween Fic a.k.a. YAHF in Buffy the Vampire Slayer fanfiction, as the events of "Halloween", where a curse transforms everyone into what they had dressed as for the night, 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..
  • An Average Everyday Supergoddess focuses on a secondary character, Harmony Kendall, whose transformation for reasons unknown doesn't reverse at the end of the night, leaving her permanently stuck as Amora the Asgardian Enchantress.
  • A Spark of Genius: Xander dresses as a "gentleman adventurer" and is transformed into a Girl Genius OC — an Agatha and Gilgamesh kid, specifically — and a Spark.
• Chaos Effect: The first Halloween Filler Arc sees the people of Domino City (specifically those who bought villain costumes) being transformed into their characters by a self-righteous fanboy who thinks that defeating them while playing the role of the hero will somehow break society's obsession with Rooting for the Empire. Edwin is less than impressed with this, least of all because he recognizes it as a Whole-Plot Reference to Buffy the Vampire Slayer.
• In Happy Frightmares, Gaz steals Dib's Graboid costume to win a school costume contest, and additionally pranks him by leaving him naked in public in the process. Unfortunately for her, Jack Skellington overhears all this and punishes her by transforming her into a real Graboid, blanking out her mind and leaving her to go on a rampage across the city, leading to her being caught and carted off to be experimented on. Then the magic wears off, changing her back to normal... and leaving her naked on live television.
• Hero Chat: One of the side stories sees Team Miraculous face an akumatized villain with this power while attending a fan convention. As they were wearing cosplay while already transformed, they end up with the abilities of whoever they were dressed as alongside their usual powers — for instance, Honeybee gets to use Katsuki Bakugo's Explosion Quirk, and Roi Singe dual-wields his staff and Lance's Bayard.
• It All Started with an OSHA Violation: Played for Horror in both instances. As Skid and Pump went down the elevator shaft in their respective costumes, Skid's skeleton costume fuses with his bones while Pump's costume head fused with his head and became an actual pumpkin. Monster was revealed to have suffered the same fate once he fell down the same shaft; the lemon head of his mascot costume became his actual head.
• Not the intended use (Zantetsuken Reverse): In "OMAKE 3 Halloween", a fairy threatens everyone attending the party that she'll turn them into their costumes, but everyone decided that For Halloween, I Am Going as Myself, and just stares at her.
• Oversaturated World: In "Living Nightmares", Twilight performs an experimental costume-related spell that results in the creation of Darth Rarity, Sweetie Skywalker, Chief Engineer Applebloom 'Scotty' Scot, Invader Spike, and Rainbow Caboose.
  Sunset: "What have we learned, Twilight?"
  Twilight: "Don't take magical inspiration from Peppy the Vampire Stabber?"
  Sunset: "I meant test new spells in a controlled environment, but that's probably a good rule of thumb as well."
• RainbowDoubleDash's Lunaverse: Costumes revolves around this trope. In an attempt to promote her wares, Rarity uses a Zebrican ritual that she thinks will make her costumes more realistic. Unfortunately, in something of a Running Gag for the Lunaverse, she misunderstands the ritual's function — it was designed to allow a zebra to take on the physical form and powers of a totem. So everyone wearing one of Rarity's costumes (and most of Ponyville was wearing one of Rarity's costumes) is turned into whatever they were dressed up as, and gains whatever abilities they subconsciously believed their new form should have. Further, the spell causes all victims to believe that they actually are whatever they'd been dressed as. Luckily, Trixie and Twilight had made their own costumes and are able to dispel the curse. But not before several adventures occur...
  • Rarity is turned into a vampony and gains hypnotic eyes and blood-sucking fangs.
  • Snips and Snails had dressed as Captain Equestria and Spiderstallion, while Raindrops was in disguise as Princess Platinum. When Raindrops!Platinum is kidnapped by a pack of diamond dogs (formerly the workers of Sweet Apple Acres), the two superheroes rescue her. Snips!Captain even gets a kiss out of it... right as the magic's dispelled, so Raindrops ends up kissing Snips.
  • Cherilee, now Catmare, attempts to rob a jewelry store before running into Batstallion (formerly Heavy Roller) and Robin (formerly Scootaloo). They successfully catch her after an innuendo-laden fight, only for Carrot Top to show up transmuted into Poison Ivy. Carrot!Ivy takes Batstallion prisoner with her spores, expressing an interest in feeding him to her plants. Cherilee!Catmare can't have that, as she has her own designs on Batstallion's body. The ritual comes undone right in the middle of the final fight between Catmare and Robin on one side, and Poison Ivy on the other.
  • Lyra and Bonbon had dressed up as Bonnie and Clydesdale. In that guise, they successfully rob the Ponyville bank, leaving the teller tied up behind them. They just finish enjoying some celebratory sex when Dinky!Holmes and Ditzy!Watson show up and incapacitate them. Dinky!Holmes then rounds things off by apprehending the teller, who'd been embezzling from the bank. That chapter ends with Bonbon worriedly rubbing her abdomen after the magic is reversed.
• This Bites!: In the third Halloween special, the Straw Hats (and many allies) are transported to Halloween Town from The Nightmare Before Christmas by B.R.O.B., and this trope ensues. Though while physically changed, their minds remain intact, so it's mostly played for laughs.
• You Call That a Costume? features Sci-Twi trying to make the Main 7's costumes more realistic, only for her spell to make them too realistic: Sunset becomes Starswirl the Bearded, Rainbow Dash becomes a werewolf, Fluttershy becomes a giant vampire bat, Rarity becomes a cowgirl, Applejack becomes a debutante, Pinkie Pie turns into Hyde-like persona Pinkamena, and Sci-Twi gains the form of Midnight Sparkle.

Films — Animated

• Monster Family: After Emma mistakenly orders monster costumes for her family from Dracula instead of the costume store she intended to call, and following a spell from Baba Yaga, each member of the family is transformed into the monster they were dressed as — Emma into a vampire, her husband Frank into Frankenstein's Monster, her son Max into a werewolf and her daughter Fay into a mummy.

Films — Live-Action

• Clown: Kent McCoy, upon donning a Clown costume to stand in at his son's party, goes from a loving father to a child eating demon under the influence of the "costume" which turns out to be literal skin and hair from an ancient European demon.
• Halloweentown II: Kalabar's Revenge: Kal plans on turning every human who dressed as a monster into that monster (and conversely to turn monsters into humans). He even has Gwen wear a horrific mask so that she would become a terrifying, hag-like goblin.
• Hallows End: The characters become their costumes when they die.
• The House That Dripped Blood: In "The Cloak", actor Henderson purchases a vampire's cloak to add verisimilitude to role as a vampire, only to discover he transforms into a vampire whenever he dons it.
• Night of the Demons 3: The demonic Angela Franklin turns characters into their demon, Cat Girl, and snake-armed gorgon costumes.
• Onibaba: The mask worn by a samurai warrior, and later one of the female protagonists, is implied to have this effect.
• The Sorcerer's Apprentice has The Dragon Horvath turning a Chinese dancing dragon into a real dragon.

Literature

• In "The Cloak", by Robert Bloch, the title garment gave the guy who was wearing it vampiric urges. Unfortunately, his Love Interest bought hers from the same guy.
• Doctor Who Expanded Universe: In 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.
• Flossie Teacakes: A particularly fabulous coat that Flossie 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.
• Goosebumps: The Haunted Mask and its sequel (also adapted for TV) has this as a plot point, in which the mask the victims wear in each book can only be removed by a symbol of love.
• Jackrabbit Messiah: There's a brief bit where the hero (and his Helpful Hallucination) have to Journey to the Center of the Mind. While there, Jack's costume is no longer just a costume!
• Magic: The Gathering: In the Ravnica Cycle 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.
• Spinetinglers: The concept of book 26 (Wear and Scare) is that kids buy some costumes from a mysterious shop, and eventually they turn into the creatures they are dressed as.
• 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?", due to each costumed person technically being turned into what they perceived their costume as being — as such, two people in the same exact costume would still turn into different things based on their knowledge and interpretation of the character they had come as, how "in-character" they were, what they were thinking or doing at the moment they transformed, and so on. There is also the "clothing curse", which affected some, but not all, of the transformed people and caused any clothing they put on to transform in the closest equivalent that would be "appropriate" for their character to wear^note so for instance anything a Jedi would put would transform into a set of robes. Stories tend to be either furry or Welcome to the Real World.

Live-Action TV

• Buffy the Vampire Slayer:
  • "Halloween": A powerful spell transforms everyone in town into whatever they had dressed as for the night — Willow becomes a ghost, Xander a trigger-happy soldier and Buffy a hapless 18th-century noblewoman. Only the costumes from one specific shop, the one run by the guy who did the spell, do this — regular Halloween costumes have no effect on their wearers at all.
  • "Fear Itself": Exploited; the gang attend a college Halloween party and all of them dress up as someone or something that would actually be useful should history repeat itself. This time, though, it's just the Halloween decorations at the party — spiders, bats, skeletons, peeled grapes posing as eyeballs — that get brought to life.
  • Angel: The same trick is pulled on a convention in the later comics, turning a bunch of nerds into superheroes and villains, and turning Spike into Angel. Or, at least, Angel as Spike sees him.
• Community: Abed is probably worth an honorable mention. He dresses up as Batman the Halloween episode "Introduction to Statistics", and although technically he never actually turns into Batman (despite appearances, it's not that kind of show), he commits to the part to such a degree that some of his friends start to think he actually believes it. But then, he is known for his rather unique view of reality.
• Misfits has an unusual example bordering on an inversion: One character wears a gorilla costume to a costume party. In the end it turns out that he was a gorilla with the superpower to appear human, leading to a scene in which he removes a gorilla mask to reveal a gorilla face underneath.
• My Babysitter's a Vampire: In one episode, Rory puts on an ancient magical mask that gives him the power to turn people into their costumes. Unfortunately, the mask also takes control of his mind, and starts building an army. To make matters worse, the mask can only be removed after the wearer is blasted with magical lightning — something WAY out of the league of the resident wizard, especially since his Old Master witch grandmother is out of town.
• Sir Arthur Conan Doyle's The Lost World: In one episode, the heroes take shelter from the rain in a castle. When they decide to change out of their wet clothes into the formal outfits they find, they become possessed by the original wearers, who were evil. Fortunately, Challenger didn't do this, leaving him free to save the day.
• Spooksville: The Halloween costume version is used with a boy who keeps trick-or-treaters hostage as more-or-less real versions of their costumes. The resolution hinges on one of the characters being able to use part of his costume that had been a toy but is now real.
• That's So Raven: In the Halloween episode, Raven and Chelsea use a wishing spell to win the costume contest at a Halloween party. However, a mistake during the spell casting also causes them to turn into cows. Despite becoming actual cows at the party, they get voted as having the best costumes.
• The Twilight Zone (1959) has "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 save for some fare to take the train home. At midnight, he dies, and they shed the masks... and discover their faces have taken on the shapes of the masks permanently save the good-hearted millionaire, who retains his human face. It should be noted this isn't set on Halloween, but Mardi Gras in New Orleans.

Mythology and Religion

• Some tribal cultures have rituals in which a mask causes the wearer to become the god or spirit it represents.
  • This is especially true in topeng, the art of Balinese masked dance-drama. The mask is extremely important, and the actor must consecrate him/herself and prepare carefully for any masked role.

Tabletop Games

• 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

• Costume Quest: This is a gameplay mechanic. You play a kid who has to rescue their sibling from evil goblins using powers channeled through their costume; when a battle begins, you transform into a Kaiju-sized version of what your costume was supposed to represent. Your starting costume, for instance — a blue cardboard box that's supposed to be a robot — turns you into a Giant Mecha.
• Digimon World: The player can find a Monzaemon costume in Toy Town. If their partner Digimon is a Numemon, it can climb into the costume and take on Monzaemon's form. Since Monzaemon is an Ultimate-level Digimon (and is, in fact, required to gain access to Toy Town's main building), this is effectively a free Digivolution, in a game where meeting the requirements to Digivolve your partner to the Ultimate level at all can be tricky.
• Five Nights at Freddy's 3: William Afton's ultimate fate, where it is revealed he suffered a grievous, painful death and became entombed in the very same suit he used to commit his crimes, his restless ghost haunting it to this day.
• Kid Chameleon on the Sega Genesis as the main gameplay mechanic. The eponymous Kid Chameleon gains different powers and costumes from various mask power-ups found throughout the game.
• The Legend of Zelda: Majora's Mask: The Deku, Goron, Zora, and Fierce Deity masks are much more than facial accessories — they contain something of the people who died to make them, and donning them turns Link into a member of their species (for the first three) or a demigod of sorts (for the Fierce Deity mask).
• I=MGCM: In the collaboration event with The Quintessential Quintuplets, when the 6 heroines get the quintuplets' and Raiha's cosplay dresses, their personalities become like the quintuplets and Raiha.
• Overwatch: Most legendary skins give the characters additional lines and/or change existing ones to fit the skin's theme.
  • Hanzo's Lone Wolf and Okami^note Japanese for "wolf" skins are very good examples, as they not only replace all dragon-related lines with wolf-related ones (i.e. "See through the dragon's eyes" is changed to "See through the wolf's eyes"), but they also change the dragons that spawn from his Ultimate into wolves, complete with a chorus of howls.
  • Another good example is Zenyatta's Cultist skin from the 2017 Halloween event, which turns him into a mechanical Cthulhumanoid. His voice lines are normally very tranquil and reflect his insightful and religious nature, but with this skin become darker, more menacing, and bring eldritch themes to mind — "the Iris embraces you" becomes "the Iris consumes you", "experience tranquility" becomes "experience... nothingness". Also, the three pairs of holographic arms that appear during his ultimate ability are replaced by three pairs of jointed mechanical tentacles.
  • Symmetra's Dragon skin, also from the Halloween event, visually makes her resemble a half-draconic sorceress, changing her weaponry to gain more fantasy-style armor plating and moving, reptilian-looking eyes. She gains some appropriately demonic and fiery lines, and all her ability lines have been changed to do in the scientist, now referring to her Sentry Turrets as "Sentry Wards," her Teleporter as a "Teleportation Portal," and her Shield Generator as a "Shield Well."
• Sunless Sea: There is an island called Visage where all who visit the island must don a mask to go past the dock onto the island. Those who wear the mask take on the role of the mask, both mentally and physically. For example, those wearing the frog mask will only croak when they try to speak, someone wearing a locust mask will act like a locust, and while wearing the mask of the moon-moth it's mentioned "You briefly flicker your wings". However, once someone removes their mask they immediately become the person they were before.
• Tales Series:
  • Tales of the Abyss: Anise can be given special dolls from other Tales or Namco games (such as Presea from Tales of Symphonia and ) which sometimes prompt Anise's end battle quote to sound nothing like anything Anise would say and sometimes end up as nonsequitors, because she's channelling the character in question. They also allow her to use their special abilities (ie, Beast for the Presea doll)
  • Tales of Vesperia:
    • Played with in a skit in the PS3 version, 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.

Webcomics

• High School Lessons: a student enchants a mask to turn him into a monster for Halloween. Unfortunately, his monster form has more eyes and tentacles than his brain can handle.

Web Original

• From the satirical Venezuelan blog Chigüire Bipolar's article Boy disguises as National Bolivarian Guard and hits his mother, grandmother and doggy, both the title's boy and his sister who was dressed up as a member of radical opposition.
• Crash Zoom: This is the premise of the third episode. Kate, bored at a LARPing event, transforms everyone into their costumes and summons a huge monster for them to fight.
• In Seanan McGuire's Patreon-exclusive short story "Face Your Furs", an unnamed theme park implants nanotechnology into the Goofy Suits employees wear to turn them into Beast Men. This is all perfectly legal, but they're still treated more like property than employees, and complications arise when the transformation turns out to be irreversible.
• Fenspace: A possible result of handwavium accidents is costumes getting incorporated into their wearers' physiology. The most drastic example to date is a guy who built a Transformer using handwavium and got shot by a police sniper after being mistaken for a terrorist. He passed out in the cab, with a burst pipe leaking handwavium into the wound, and woke up with his mind now in the Transformer body. Likewise, the first catgirl was created when a con-goer in a handwavium-enhanced catgirl costume ate food contaminated with Guacamole (a strain of 'wavium specialized for bio-modification), and another guy became a cyborg when he accidentally drank a bottle of handwavium while wearing a suit of Powered Armor.
• FNAFNG: During "Space Furries", or the AU's version of Five Nights at Freddy's: Security Breach, Vanny's costume will sometimes give her the ability to morph into an anthropomorphic bunny rabbit depending on how strong Glitchtrap's hold is on her.
• Ink City: Discord inflicts this upon the characters when he crashes their Halloween party. This includes several residents temporarily transforming into each other thanks to their costume choices, like Rigby becoming Trevor, and Yakko becoming his own player.
• TLF Travel Alerts: For Halloween 2017 they warned of a student who dressed as a circle line train in 1997 who became an example of this trope and was still running in service. Consider your costumes wisely!

Western Animation

• Batman: The Brave and the Bold: Batman turns into a man-bat hybrid, Bronze Tiger into a tiger-human hybrid and Vulture, Fox and Shark turn into, you guessed it, subjects of Animorphism.
• Danny Phantom: In "Reality Trip", a Reality Gem transforms a trio of nerds at a comic convention. Unfortunately for Danny, they were dressed as supervillains.
• Darkwing Duck: In "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. The gas doesn't actually affect our hero, though, because he came to the party as himself.
• The Fairly OddParents!: Timmy wishes for this to happen on Halloween — unfortunately for him, his wording of the wish prevents him from unwishing it. He wanted everyone's costumes to be "real and scary"... and his fairy god parents just had to dress as a "floating human janitor" and a "floating human nurse". Even worse, the popular kids were dressed as Crash Nebula villains capable of forming a Doomsday Device that can blow up a planet.
• Lilo & Stitch: The Series: In "Morpholomew", after capturing the title Experiment, Lilo decides his "one true place" is at the local costume shop, where he uses his powers to physically transform people into whatever character they want to dress as.
• The Magic Key: One episode plays this trope to a T. The children are about to go to a fancy dress party when, like in every episode, they are sucked into a magical world via the Magic Key. In this one they meet a farmer whose crops are being eaten by a group of rooks, led by an anthropomorphic leader called the Rook King. When the children try to help the farmer, the Rook King turns them all into their costumes using his staff and they have to work out a plan to defeat him whilst trapped in their new forms. Once they capture him and get hold of the staff, they turn back to normal.
• Mr. Benn: The premise of the show has Mr. Benn (a bowler-hatted Everyman) visit a magical costume shop, try on a costume and become that character — spaceman, cowboy, prisoner, clown... — for the episode.
• Rugrats: In "Curse of the Werewuff", Angelica convinces the babies that they will be transformed into their Halloween costumes.
• The Powerpuff Girls (2016): This happens to Mojo Jojo, dressed as a cat for Halloween, in "Witch's Crew", thanks to Princess Morbucks's potion she originally used to make herself more beautiful and powerful than the titular heroines, which backfired thanks to their intervention. In the process, Morbucks was turned into an ogre and the Powerpuffs into evil witches. Mojo and Morbucks are then forced to work together to reverse the spell.
• Punky Brewster: Of a non-Halloween variation, in "Fish Story", Punky is vying for the role of a mermaid in a school play, but her costume is quite slipshod. Glomer tries to help out by turning Punky into an actual mermaid.
• The Simpsons:
  • In the third segment of "Treehouse of Horror XVI" ("I've Grown a Costume To Your Face"), a witch curses everyone in town by making them become their costumes, except for Hans Moleman who got turned into a mole despite not wearing a costume.
  • One of the Halloween comics had this happen in messy fashion when Rod and Todd Flanders became possessed by demons, including a lethal How Do I Shot Web? when Comic Book Guy, dressed as Superman, tries to test out his X-ray vision on stuff in his pocket and sets his eyes to heat vision instead.
• Teen Titans Go!: In "Monster Squad!", the Titans decide to be the scariest trick-or-treaters in town, so Raven turns them into real monsters.
• Ultimate Spider-Man: In the Halloween Episode "Strange Little Halloween", Baron Mordo casts a spell that transforms trick-or-treaters into the monsters they're dressed as. Non-monstrous costumes are interpreted more ... creatively ... so they still turn into rampaging monsters (astronaut into alien monster, princess into zombie princess). The spell is also pretty liberal in its definition of "costume" — Spidey is shielded by protection magic Dr Strange cast on him in a previous episode, but Ant-Man's helmet is enough for him to turn into a giant ant.
• Witchs 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.

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