Fierce Deity Link

The Fierce Deity is one of the transformations Link turns into in, achieved by wearing the Fierce Deity's Mask. This transforms him into a mysterious and powerful warrior resembling a Hylian. It is one of four mask transformations in the game.

Transformation
Link obtains the Fierce Deity Mask after he trades the twenty non-transforming masks to the four Moon Children inside the Moon. The final child sitting by the tree will give Link the Fierce Deity Mask after noticing all the other masks are gone. Wearing this mask enables Link to transform into the Fierce Deity, however only during boss battles unless certain glitches are manipulated.

Abilities
In the Fierce Deity form, Link gains greater strength and range in his attacks as well as certain new abilities. 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 superior power. Link can also use the sword to release deadly sword beams that burn enemies with each targeted swing.

Majora's Mask manga
Link had received the Fierce Deity's Mask from Majora and became Fierce Deity Link during his fight against it. In a flashback of Majora, it is revealed that while as a dragon, a mysterious man bearing a strong resemblance to the Fierce Deity in both appearance and clothing had visited Majora. The man played music for the dragon, who danced for three days and three nights before dying. 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. Series
Fierce Deity Link appears as a Sticker in. He also appears as one of Link's alternative costumes in.

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 Majora's Mask.

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 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 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.
 * The Fierce Deity form is popularly referred to as "Oni Link" within 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 without the use of glitches. One method to comparing sizes is to create placeholder statues via the "Elegy of Emptiness" during boss fights: the statues are perfect replicas 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.