Evil Mask

A more literal version of Becoming the Mask, this is a mask, helmet, or other headgear that has some form of corrupting influence on its wearer. It may be possessed by an evil entity, a Mind Control Device controlled by the Big Bad, a strange alien parasite... whatever the specifics, it is powerful enough to turn most ordinary people evil or drive them insane. Often grants the wearer supernatural powers as well, which of course reduces the victim's temptation to remove the mask.

The mask may have an immediate, obvious effect, or it may take a while before the wearer's friends notice he is Not Himself.

Bonus points if you can't get it off.

Sub Trope of Mask of Power and Clothes Make the Maniac. See also Clingy Costume.

Anime and Manga

 * Ichigo  hollow masks in Bleach. Although once he's beaten his inner hollow into submission it becomes just another powerup

Comicbooks

 * One of these turns out to be the villain of the Ghost / Hellboy crossover comic.
 * The Mask obviously has one. It's more evil than its film counterpart.
 * A trilogy of Italian Mickey Mouse story arcs has Mickey and Goofy transport to a fantasy-esque dimension, where they help overthrow the Big Bad The Lord of The Mists, who returns in the third arc, only for it to be revealed that its his helmet being worn by a random passersby who had found it in the ruins of the Lords castle, and had his personality overwritten by the helmet.

Film

 * In the film version of The Mask, the mask was really only as evil as its wearer.
 * In Onibaba, the mysterious samurai wears an intimidating demon mask, but it isn't until Kichi's mother steals it that we begin to suspect it harbors a malicious curse.

Literature

 * The Haunted Mask from the Goosebumps series of books had a Halloween mask that made it child's play to frighten people, but altered the wearer's personality and became progressively more difficult to remove.
 * And then the sequel had another one appear, then reveal that the entire shop was filled with these--none of the masks weren't evil or sentient.
 * It Got Worse in the live action adaptation of the sequel. In which even masks that weren't themselves evil made the wearer vulnerable to mind control from the more powerful ones.
 * In the Friday the 13 th book series, people become Jason when they put on his mask.

Live Action TV
"Giles: (mimicking Joyce) Do you like my mask? Isn't it pretty? It raises the dead. Americans."
 * There was an episode of Buffy the Vampire Slayer with an evil African mask that Buffy's mom had hanging in her room. Unlike most examples, it could start summoning zombies and causing havoc just by being there. Once one of the zombies reached it and put it on, however, things got even worse.

Tabletop Games

 * Call of Cthulhu supplement Fragments of Fear, adventure "Valley of the Four Shrines". A metal mask with Cthulhu's face grants the wearer powers such as being able to breath and see underwater. However, each time it's put on the wearer must make a SAN roll or lose sanity. Once their SAN reaches 0 they become a devout slave of Cthulhu and are subject to his mental commands.
 * Warhammer Fantasy has the Crown of Sorcery, which is this trope but with a crown rather than a mask. It contains a portion of the Supreme Lord of the Undead's undying spirit, and confers some of his magical powers as well as acting as a vector for his corrupting influence. Strong-willed or single-minded wearers, such as the man-god Sigmar and the Orc Warlord Azhag the Slaughterer were merely influenced and addled by the crown, but less willful wearers such as the beastmaster Kadon were entirely subverted.

Toys

 * Infected Masks and Krana in Bionicle.

Video Games

 * Majora's Mask. That Skullkid wearing it might seem like a monster, given the various gruesome curses he inflicts on you and the residents of Termina - but then you learn that before he put it on, he was just a lonely kid who wanted friends. By the end of the game, it becomes clear that the mask itself has been pulling the strings the entire time, and has no motive for any of its cruelty besides its own amusement.
 * The Legend of Zelda Majoras Mask also had the Fierce Deity mask, which was implied to contain dark powers nearly as powerful as Majora. Link didn't seem to have a problem with it, though, probably because he's, well, Link.
 * The mask of Clavicus Vile in The Elder Scrolls.
 * The Carnival of Shadows masks in City of Heroes; the original, owned by the Carnival's leader, is the Soul Jar of a woman who led a similar group in 17th century Venice.
 * Kane's mask in Shining Force is a mind-controlling device created by the Big Bad Darksol. Defeating Kane causes the mask to crack and fall off, returning Kane to his former self.
 * The Snow Queen Mask in the original Persona.
 * The third Crash Bandicoot game introduced Aku Aku's Evil Twin Uka Uka, who was retconned into being the series Big Bad. He reprised this role, as well as being a powerup, in Crash Team Racing. However his position has shifted back towards Cortex as of late.
 * The Pokémon Yamask carries a mask that will possess anyone who wears it.
 * The Mask of Dark Earth from Sly Cooper.

Western Animation

 * He-Man and The Masters of The Universe had an episode entitled "Masks of Power". Given the trope page, the plot is pretty obvious.
 * Jackie Chan Adventures season 4 featured the nine Oni Masks, which held the spirits of nine demon generals who commanded nine different tribes of Shadowkhan (the heroes having only faced one type throughout the past three seasons). Anyone who wears an Oni Mask becomes increasingly demonic, although a good person can hold off the effects unless they're forced to summon Shadowkhan.
 * In Adventure Time,  post "Holly Jolly Secrets".