The Fierce Deity's Mask (Japanese: 鬼神の仮面 Kijin no Kamen) is a 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.
It is given to Link when he "trades" all his 20 normal masks to the children representing Majora's Mask on the moon. When Link dons the mask, he transforms into an extremely powerful warrior called 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. Link also can use any other items besides his other three transformation masks.
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.
Majora's Mask Manga
The Fierce Deity's Mask makes an appearance in the Majora's Mask Manga, where 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 does land one attack on Link, but the attack has no effect him, Majora starts off the battle in its Majora's Incarnation form), 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 then making him appear somewhat more aggressive, since he attacks the moon kids with no provocation.
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 statement of Majora's insanity, and that Majora thinks of himself as a hero.