Fierce Deity's Mask

The Fierce Deity's Mask (Japanese: 鬼神の仮面 Kijin no Kamen) is the final transformation mask that Link can obtain in The Legend of Zelda: Majora's Mask. Nobuyuki Hiyama, the voice actor of adult Link in The Legend of Zelda: Ocarina of Time, also voices Fierce Deity Link. The model for Link as a Fierce Deity is exactly the same one used for Link as an adult in Ocarina of Time, only with a different design and a different sword. However, the Fierce Deity model is much taller than Adult Link, possibly 8 feet (2.4 m) at the least.

The Legend of Zelda: Majora's Mask
It is given to Link when he "trades" all his 20 normal masks to the children representing either Majora or the Happy Mask Salesman on the Moon. When Link dons the mask, its power is activated and he transforms into an extremely powerful warrior known as Fierce Deity Link with greatly enhanced abilities and powers. While wearing the mask, Link is limited to wielding a giant, two-handed double helix sword that is capable of firing beams of magical energy at Z-targeted enemies, at the expense of magic. However, Link cannot use any other items besides his other three transformation masks, although repeated attempts to use a bottle may be successful.

Due to its immense power, the mask can only be worn in confrontations against bosses. Otherwise, it is not available for use, much like the Giant's Mask, unless the player applies glitches or cheat codes to the game. Exactly why Majora ever gave Link something that could allow him to kill it so easily is unknown, although Majora may have been so insane and twisted with power that he thought that he was stronger than the the Fierce Deity's Mask.

The Legend of Zelda: Majora's Mask manga
The Fierce Deity's Mask makes an appearance in the Majora's Mask Manga. Here, the psychotic Majora gives the mask to Link. Although he's warned not put the mask on, Link is far too disgusted by Majora's evil to listen.

After putting on the mask, Link attacks and effortlessly kills each of the moon kids (each one was wearing a boss mask), and after a brief battle with Majora, in which Majora, in its Majora's Incarnation form, does land one attack on Link, but the attack has no effect on him and angered, Link tosses Majora against a wall. Majora then assumes its third form, Majora's Wrath, and tries to attack Link, but Link kills it with one swing. Despite warnings from others, wearing the mask didn't have any effects on Link's personality, other than making him appear somewhat more aggressive, since he attacks the moon kids with no provocation.

Possible history
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 himself 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, and 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. 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 being Darmani, and the Zora Mask being 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 The Legend of Zelda: Twilight Princess (though this could be coincidence, as the way Link sees the Interlopers might not be their real appearance). It could perhaps be that the Fierce Deity was a war god that the Interlopers worshiped before they were banished to the Twilight Realm. Since he would have aided his worshipers 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 the Fierce Deity is Link's Terminan counterpart, due to the resemblance between the mask and Link.