The Legend of Zelda: Majora's Mask/Characters/Fierce Deity Link

Fierce Deity Link is an alternate form of Link in The Legend of Zelda: Majora's Mask. Unlike Ocarina of Time, Link doesn't need to wait seven years in order to assume an adult form, but instead acquires a rare mask named the Fierce Deity Mask. This mask turns young Link into an incredibly powerful, nigh-invincible demonic god warrior armed with an enormous double-helix sword capable of firing light beams. A Gossip Stone reveals that the mask contains the essences of every mask, the reason for its unrivaled power surpassing even that of Majora's Mask. Fierce Deity Link is also restricted from using any items besides bottles (which have strange effects when utilized). However, the use of this incredibly powerful mask is limited to boss battles only, although there is a glitch within the game that clever players can take advantage of in order to use it outside of boss battles.

Acquiring the Mask
The Mask can be obtained by collecting all twenty normal masks in the game. Next, the player will need to reach the inside of the Moon. Here, players will need to give all twenty of their masks (excluding the transformation masks) to the children wearing the masks of the various bosses the player has defeated. Passing the hide-and-seek games with every child allows the player to talk to the kid wearing Majora's Mask under the giant tree, who then gives Link the Fierce Deity Mask.

Kishin Link
Fierce Deity Link is also referred to as "Kishin Link", or alternately "Oni Link" (the latter is less accurate because it is only the translation of the first of the 2 kanji used (鬼), usually pronounced "Oni"). "Fierce Deity" is basically a direct translation of Kishin (鬼神), although "Deity" is used in place of the more usual "God".

Methods of becoming Fierce Deity Link outside of Bosses
Link can become Fierce Diety Link by going into water where young Link can swim and Zora Link stands because of height, then diving under water as Young Link then putting on the Zora mask. If done correctly, the B button can be pushed repeatedly quickly. Then while pressing B button quickly, you can press the C button for the Zora Mask. If done correctly, the B button will be blank and Link is stuck swimming. In most places, you can start swimming toward the side and you will roll. Pause the game and replace the position of the Zora mask with that of the Fierce Deity mask. Now you can change into Fierce Deity Link.

Another Method involves is to do the Anju/Kafei side quest up to where you must enter Sakon's hideout. Go to Ikana Canyon and enter the hideout. Start the track and enter the maze. Put on a mask for Link (any mask) and keep it on until you switch to Kafei. Then press start and put the Fierce Deity mask on the same C button as the mask you put on Link. Then solve the puzzle with Kafei. When you swtich back, Link will be forced to transform into Fierce Deity. Complete the puzzle with Fierce Diety and you will be ejected back to Ikana Canyon. From there you can do whatever you wish with Fierce Deity (you must jump in the river and swim toward Southern Swamp in order to get to Termina Field). This glitch does not work on the European (PAL) Version or the re-release Gamecube version. Also, when completing the puzzle in Sakon's Hideout, you may get stuck in a doorway as Fierce Diety, and you will teleported outside as normal Link, though, it is very uncommon for it to happen.