Fierce Deity's Mask

The Fierce Deity's Mask (鬼神の仮面) is the final transformation mask that Link can obtain in The Legend of Zelda: Majora's Mask. The mask allows Link to transform into the avatar of a mighty god, Fierce Deity, with the capability to shoot Sword Beams. Because of this, it can only be worn during boss battles, unless glitches are used. Nobuyuki Hiyama, the voice actor of adult Link in The Legend of Zelda: Ocarina of Time, also voices Fierce Deity Link. The design for Link as the Fierce Deity is clearly based on adult Link's character model in Ocarina of Time, albeit with different colors, markings, and a unique sword. Also, Fierce Deity Link is much taller than adult Link, standing only slightly shorter than Odolwa at around eight feet (2.4 m).

Acquisition
Link can obtain the Fierce Deity's Mask by trading the twenty non-transformation masks to the four Lunar Children wearing the Boss Remains at the Moon. Ironically, Link obtains the mask from the child wearing Majora's Mask just prior to the final battle with Majora. The child gives him the Fierce Deity's Mask so that the two of them can play "good guys against bad guys" together. The Fierce Deity's Mask is not required to defeat Majora, but it makes the battle much easier. The difficulty of any of the bosses fought will be greatly reduced if using the Fierce Deity's Mask, aside from Twinmold, who is very difficult to hit with the mask on. Also, though the abilities granted by the mask can inflict great damage on Goht, the fact that he runs around the arena makes it difficult to take aim, and Fierce Deity Link is not fast enough to catch up to the mechanical beast.

Theory
Some believe that the Fierce Deity was a nemesis of Majora's, possibly the hero who sealed Majora into the mask long ago, as is implied in the Majora's Mask manga. If this is the case, then Majora likely gives Link the mask in order to defeat both Link and the spirit inhabiting the mask at the same time. This theory assumes that the Lunar Child wearing Majora's Mask is influenced by or is himself a personification of Majora.

Other theories include that the mask was given either through naivete or insanity, or that the Lunar Child is not Majora, but a manifestation of Skull Kid's gentle inner self, who gives Link the Fierce Deity's Mask so that Link can defeat Majora and save Termina.

It has been theorized that the "Fierce Deity" may be Link's Terminan counterpart, due to their similar appearance and voices. The voice similarity could be explained by the fact that Link's voice and clothing are partially preserved in his other transformations, however the Fierce Deity's Mask itself has a similar face and hat.

Unlike the other three transformations masks, the Fierce Deity's Mask uses the possessive form of the word, implying that the mask itself once belonged to a Fierce Deity, as opposed to the deity's soul being sealed inside the mask.

Super Smash Bros. Brawl
Fierce Deity Link is featured as a sticker. Its sticker gives a +21 bonus to slash attacks, and can only be applied to Link, Toon Link, Princess Zelda, and Ganondorf.

Etymology
The term "fierce deity" (鬼神) 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.