Fierce Deity's Mask

The Fierce Deity's Mask (Japanese: 鬼神の仮面 Kijin no Kamen lit. Devil God's Mask) 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 model for Link as 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 eight feet (2.4 m) tall, only slightly shorter than Odolwa.

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. Fierce Deity's Mask is not required to defeat Majora, but it makes the battle much easier.

Theory
Some believe that Fierce Deity is Majora's nemesis and that Majora gives Link the mask in order to defeat the hero as well as the spirit inhabiting the mask at the same time. This theory assumes that the Lunar Child wearing Majora's Mask was influenced or a personification of Majora itself.

Other theories state that the mask was given either through naivety or insanity, or that the Lunar Child was not Majora.