Fierce Deity Mask

The Fierce Deity's Mask is the last of the twenty-four masks that Link can obtain in. It is said to posses the merits of all masks.

Location
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 obtainable 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.

Uses
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 just as evil as Majora's Mask. In this state, Link has an adult size and voice, and has devastating attacks performed with the helix-edged sword (or its Sword Beams). A sword strike (minus 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.

Trivia

 * The Adult Link Mask was originally developed as the fourth transformation mask in the game. Near the end of development, the developers changed this mask to the Fierce Deity's Mask in order to add variety to the transformation, but still maintain an older version of Link in the game. This is evident in Link's voice while wearing the mask; it is the same voice as adult Link from Ocarina of Time. Also, in Hyrule Historia, the Majora's Mask section contains concept art of what appears to be adult Link from Ocarina of Time, practically confirming that the original idea was adult Link.
 * Digging into the game reveals a textured model of a Link mask. It is possible that it was used to turn Link back into his Hylian form when wearing other transformation masks. Concept art in Hyrule Historia shows Deku Link wearing a similar mask. It is also possible that this mask was later replaced by the Fierce Deity Mask in terms of transforming Link in an adult form.
 * 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 (demon) wears, and the Japanese name for tag is "pretending to be an oni".
 * Despite not being essential to the game's completion, Link obtaining the Fierce Deity's Mask is stated to be canon in Hyrule Historia.