Fierce Deity Link

The Fierce Deity is a transformation of Link's when equipping the Fierce Deity's Mask in Majora's Mask. This transforms him into a mysterious yet powerful warrior resembling adult Link from Ocarina of Time.

Characteristics
Fierce Deity Link is characterized by white hair, facial markings, a light blue tunic underneath a metal cuirass, metallic gauntlet, black leggings, white eyes without pupils, and very large height and width (standing two heads taller than a normal adult Hylian and only slightly shorter than Odolwa's 2.4 m).

In this form, he is unable to use any items that cannot be contained within Bottles. Because the Fierce Deity wields a large, double-handed, helix-edged sword, he cannot use a shield, although this disadvantage is compensated for by the sword's great physical power, range, and ability to release deadly sword beams that magically burn targets with each targeted swing.

Majora's Mask manga
Link received the Fierce Deity's Mask from Majora and become Fierce Deity Link during his fight against it. In the flashback of Majora, it is explained that Majora's Mask is carved from the armor of a powerful dragon, Majora. Majora guarded an empty land that was neither moving nor dead. This dragon's armor was sought after, as it was believed that the armor could grant wishes and give power. Many came hoping to claim the armor for their own reason, be them good or evil, but Majora killed them all. One day, a mysterious man, bearing a strong resemblance to Fierce Deity in both his appearance and clothing, visited Majora. This man played music for the dragon, who danced for three days and three nights before dying, supposedly allowing time to move forward. The man then carved a mask, the Majora's Mask, from the creature's hide, hoping to seal its power. It is hinted in the story that the stranger is in fact a god, the first being that is able to overcome the beast of Majora, and that it does so with music that controls the flow of time.

Super Smash Bros. Brawl
Fierce Deity Link appears as a sticker in Super Smash Bros. Brawl.

Fierce Deity Speculation
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, which is the soul. However, since Anju's quote doesn't divulge much, it is impossible to tell. The Fierce Deity was shown in the manga to corrupt even Link, leading the reader to believe the mask houses a powerful spirit, although Link did possess enough control to remove the mask at the end of the fight against Majora's Mask.

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 itself 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, implying 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. Also, when in Majora's Incarnation, Majora performs several different dances. 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 is Darmani, and the Zora Mask is 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 Twilight Princess. It could perhaps be that the Fierce Deity was a war god that the interlopers worshipped before they were banished to the Twilight Realm. Since he would have aided his worshippers 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 that the Fierce Deity is Link's Terminan counterpart, due to the resemblance between the mask and Link. This theory finds a form of support in the extra chapter of the manga, where a warrior resembling the Fierce Deity comes across the great beast from whose armour Majora's Mask is made. However, the warrior lacks the pure white eyes and facial markings seen on the Fierce Deity, and uses a weapon that takes its form from a feather, as opposed to the Fierce Diety's dual-helix weapon.

None of these theories have ever been proven to be factual; they have little to no in-game support and are fan-made speculations.

Glitches
Main Article: Fierce Deity Link Glitches.

Trivia

 * The discs of light fired from the tip of the Fierce Deity's sword when a foe is targeted are similar to the Sword Beam, an ability absent from Ocarina of Time, The Wind Waker, and Twilight Princess.
 * Link's Fierce Deity form is popularly referred to as "Oni Link" within the The Legend of Zelda community.
 * Fierce Deity Link is taller than most other characters in Majora's Mask; however, because the mask can only be used in boss rooms, it is difficult to accurately estimate the Fierce Deity's comparative size. One method to comparing sizes is to create placeholder statues via the Elegy of Emptiness during boss fights: the statues are perfect imitations of Link (and his transformations) in size and stature. Oddly, although the Fierce Deity is able to enter buildings through comparatively small doors, his tall stature usually prevents him from exiting through them, making it easy to get stuck inside.
 * Unlike Zora Link, but like Deku and Goron Link, Fierce Deity Link does not blink, most likely because of the action mostly being focused on rather than his facial expressions.

Etymology

 * The term "fierce deity" (鬼神 onigami or kishin, literally demon god) is a term referring to several deities in the Shingon Buddhist pantheon, such as Bishamonten or the Myō-ō, who are fierce and wrathful toward the negative forces that work against humanity's enlightenment, but are benevolent and supportive to humanity. They are seen as deities that work to defeat evil, a quality Link possesses in all his appearances, and which the Fierce Deity's Mask gives him greater ability to achieve. Kishin (or kijin, or onigami; they are all simply different readings of the 鬼神 kanji compound) doesn't actually equate the subject with oni or evil at all; it merely states that the deity is as fierce as an oni. The term does not carry any connotations of evil in the Japanese language.