Fierce Deity Link

Fierce Deity is a transformation of Link's. Equipping the Fierce Deity's Mask transforms Link into an adult, powerful warrior. The scheme of this form is white hair, face paint, light blue clothing, and white eyes without pupils. In this form, he is unable to use any items that can't be contained within bottles, including his shield. This disadvantage is compensated by the double-handed Fierce Deity Sword which releases magical attacks with each targeted swing.

Link's Fierce Deity form is popularly referred to as "Oni Link" within the The Legend of Zelda community: the word "oni" is Japanese for "demon", as well as the Japanese term for the "it" player in games like tag and hide-and-go-seek, which is notable considering that the child of the Moon wearing Majora's Mask asks Link to play a game, where Link is "it". However, while appropriate, this name is technically incorrect, as it is only half the translation. In the Japanese version, the Fierce Deity's Mask was called the Kishin Mask. The kanji for oni (demon) and kami (god) combined reads kishin; meaning fierce god or fierce deity.

The being within the mask
"He's a ferocious god!"

- Anju (Manga)

The Fierce Deity's past remains largely unexplained in the game, though it is likely that it is an ancient being like Majora. It is unknown if Fierce Deity is like the Goron and Zora masks, with a soul sealed inside them, or like Majora's Mask, where it is the soul. However, since Anju's quote doesn't divulge much, it is currently impossible to tell. The Fierce Deity was shown in the manga to corrupt even Link, leading the reader to believe the mask houses a very powerful spirit, even though Link did possess enough control to remove the mask at the end of the fight against Majora's Mask.

Some speculate that the mask is Link's Terminan counterpart, because the mask itself resembles Adult Link.

Who is the Feirce Deity?
Though the mask's history is never elaborated upon in the game, it is implied, through what is said by the likes of Tatl, and the fact that Majora himself gave Link the mask, that the mask was created by a Deity far stronger than Majora, and that Majora could possibly have given the mask to Link in an effort to destroy them both. The Fierce Deity's evil may be even more dark than that which resides in Majora's Mask, as Majora calls him "the true bad guy", though this may just be a lie or exaggeration on the part of the insane Majora, and that Majora considers itself a hero.

Other theories suggest that Majora is simply insane. It is likely that Majora gave Link the mask because, as it suggested, it simply wanted to play. This is a recurring theme; the fact that Majora transforms Link into a Deku Scrub when it could have killed him, the way that it chooses to raze Termina by crushing it under a moon with a face, and the fact that it gives Link the mask before their final battle, implies that Majora, rather than malicious, is in fact playful and (clearly) mischievous. The Fierce Deity's Mask could be a way of evening the odds; a sense of fair play, in that Majora and Link would be equal in terms of power.

Because the other transformation masks contained the spirit of someone, (the Deku Mask generally believed to be the Deku Butler's son, the Goron Mask being Darmani, and the Zora Mask being Mikau) it can be assumed that the Fierce Deity's Mask also contains someone's soul. Due to its name, it is possible that the spirit inside the mask is a god. In fact, the Fierce Deity bears a partial resemblance to the Dark Interlopers who later became the Twili in The Legend of Zelda: Twilight Princess (though this could be coincidence, as the way Link sees the Interlopers might not be their real appearance). It could perhaps be that the Fierce Deity was a war god that the Interlopers worshiped before they were banished to the Twilight Realm. Since he would have aided his worshipers during their war against the other races of Hyrule, he too would have been punished. In his case, the Goddesses sealed him away in a mask.

A final possible theory is the Fierce Deity is Link's Terminan counterpart, due to the resemblance between the mask and Link

Glitches
See Fierce Deity Link Glitches.

Trivia

 * The discs of light shot from the tip of Fierce Deity Link's sword when a foe is targeted are similar to the Sword Beam, a move absent from Ocarina of Time, The Wind Waker, and Twilight Princess.