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 and powerful warrior resembling a Hylian.

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. Like the other masks, it is implied to contain a powerful spirit. In the manga, the Fierce Deity corrupts even Link, although Link did possess enough control to remove the mask at the end of the fight against.

Because Majora gives Link the mask, calling it the "true bad guy", it can be theorized that Majora orchestrated the events of the game, intending for both itself and the Fierce Deity to be destroyed. However, the context in which it is given, as a child (Majora) designating Link as the oni (Fierce Deity) in a game of tag, it is more likely that Majora was simply toying with Link, or was even delusional.

As the Fierce Deity resembles a white-haired adult Link, it can also be thought that the Terminan equivalent of the enduring spirit of Hylia's hero. It has also been theorized as a lost or punished deity of other shadowy races, especially those who ended up being sealed away, such as the Sheikah, Ancient Tribe, the Ikana, the Dark Interlopers, or the Dark Tribe.

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 is used to refer 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 of humanity. They are seen as deities that work to defeat evil, a quality which the Fierce Deity's Mask gives Link greater ability to achieve.