Fierce Deity Mask

The Fierce Deity's Mask is an item in.

Location and Uses
Link can obtain the Fierce Deity's Mask by trading the twenty non-transformation masks to the four Children of the Moon inside the Moon. Since the Deku Mask, Goron Mask, and the Zora Mask cannot be traded, Link must have in his possession all of the other masks before being able to receive the Fierce Deity Mask. Once this has been done, the young hero obtains the mask from the child wearing Majora's Mask. He gives him the mask so that the two of them can "play" together. It is said to posses the merits of all masks.

The Fierce Deity's Mask is one of the four masks that have the ability to transform Link into a different body. When obtained, it is suggested to be as dangerous as Majora's Mask. In this state, Link has an adult size and voice, and attacks with a helix-edged Sword which can release Sword Beams. A Sword strike (minus the Sword Beam) deals the same amount of damage as the Great Fairy's Sword.

The Fierce Deity's Mask may only be used during boss battles, with the exception of glitches.

Hyrule Warriors
In, the Fierce Deity's Mask appears as a type of Badge in the Bazaar. It also appears as part of Young Link's moveset, where he can temporary transform into the Fierce Deity.

Trivia

 * Despite not being required to complete the game, Link obtaining the Fierce Deity's Mask is stated to be canon in.
 * In the Japanese text of the game, the child wearing Majora's Mask specifically states that he wants to play tag with Link. This is a play on words as the Japanese text states that the Fierce Deity Mask is a mask an oni (a type of Japanese ogre) wears, and the Japanese name for tag is "pretending to be an oni."
 * Early in development, an Adult Link Mask was meant to appear in the game, but was later replaced by the Fierce Deity's Mask. This was probably due to the fact that Link can now use items that only Adult Link could in, rendering the mask useless. In Hyrule Historia, the Majora's Mask section contains concept art of Adult Link, confirming that Adult Link was meant to appear sometime during development.
 * Inside the game's code, a textured model of a Link mask can be found. It is possible that it was used to turn Link back into his Hylian form when wearing other transformation masks, as concept art in Hyrule Historia shows Deku Link wearing a similar mask.