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.

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.