Fierce Deity Link

Fierce Deity is a transformation of Link's. Equipping the Fierce Deity's Mask transforms Link into an adult, powerful warrior. The scheme of this form is white hair, face paint, light blue clothing, and white eyes without pupils. In this form, he is unable to use any items that can't be contained within bottles, including his shield. This disadvantage is compensated by the double-handed Fierce Deity Sword which releases magical attacks with each targeted swing.

Link's Fierce Deity form is popularly referred to as "Oni Link" within the The Legend of Zelda community: the word "oni" is Japanese for "demon", as well as the Japanese term for the "it" player in games like tag and hide-and-go-seek, which is notable considering that the child of the Moon wearing Majora's Mask asks Link to play a game, where Link is "it". However, while appropriate, this name is technically incorrect, as it is only half the translation. In the Japanese version, the Fierce Deity's Mask was called the Kishin Mask. The kanji for oni (demon) and kami (god) combined reads kishin; meaning fierce god or fierce deity.

The being within the mask
"He's a ferocious god!"

- Anju (Manga)

The Fierce Deity's past remains largely unexplained in the game, though it is likely that it is an ancient being like Majora. It is unknown if Fierce Deity is like the Goron and Zora masks, with a soul sealed inside them, or like Majora's Mask, where it is the soul. However, since Anju's quote doesn't divulge much, it is currently impossible to tell. The Fierce Deity was shown in the manga to corrupt even Link, leading the reader to believe the mask houses a very powerful spirit, even though Link did possess enough control to remove the mask at the end of the fight against Majora's Mask.

Some speculate that the mask is Link's Terminan counterpart, because the mask itself resembles Adult Link.

Glitches
A glitch was discovered that became very popular because it allowed the player access to the Fierce Deity Mask outside Boss Battles. The glitch involved part of the Kafei and Anju quest, specifically, Sakon's Hideout; although it wasn't necessary to actually do the entire quest by using a glitch in the map design. On the third day at any time the player could go to Sakon's hideout and gain access to it by putting on the Bunny Hood and rolling at a crack on the right side of the boulder blocking the entrance to the Hideout. After a few tries Link would pop through the side and the player would have to immediately hold left on the control stick to avoid dying. If done correctly you would walk into Sakon's Hideout as if you were doing the quest normally. While still wearing the Bunny Hood, the player needed to resume the quest as if they had been doing it all along. Once the player had taken control of Kafei, it was possible to open the Start Menu, go to the mask screen and place the Fierce Deity Mask over the Bunny Hood on the C-Buttons. Once control was returned to Link, he would immediately put the mask on and transform like normal. Once this was accomplished, all the player had to do was continue the hideout puzzle normally and would eventually be outside in Ikana Canyon. While wearing the mask it was not advised to talk to any person who responded to Link differently depending on what form he was in, since it would cause the game to freeze.

At first, it was believed that after getting into Clock Town, the player had to reset the game to continue playing, since the Guards would never let you pass even if the player spoke to them. This was eventually proved wrong by preparing an escape route before attempting the glitch. All you needed was: a Magic Beans, access to the Bombers' secret hide-out, and the balloon that blocked passage to the telescope to be popped. The bean needed to be planted and grown on the Soft Soil outside the Observatory by the fence. Once all this was set up, it was possible to escape Clock Town and return to exploring. Another way to escape is by sidestepping backwards fast enough to push the guard out of the way. This can also be done with Deku Link at the beginning of the game. Jumping into the water in the Laundry Pool or in the basement of the Clock Tower will also allow escape, or any body of water, as it will enable transforming into Zora Link.

This glitch has been exterminated and can only be used in the original Nintendo 64 American and Japanese versions since pausing as Kafei is no longer allowed. The glitch was fixed in the Nintendo 64 European release and was also fixed in the Collector's Edition for the GameCube.

The second glitch
A more simple way to transform was eventually discovered. By riding Epona to Milk Road or Southern Swamp, it was possible through a saving glitch to get all the C-Buttons on the horse. There was a chance that when the game was reloaded, you would start on her, and have access to the C-Buttons and whatever was assigned to them, although the icons weren't visible. Unfortunately, if the Fierce Deity Mask was equipped straight after loading, you would have to wait until the Dawn of The Second or Third day to get off Epona, due to conflict between Fierce Deity Link's function of keeping his sword out, and the bow being assigned to B on Epona. By riding to the Gorman Brothers' race course and challenging them, you could avoid this little problem. Once the race started, you could safely put the mask on, and after it had ended, the bug would be canceled out, and you would be standing, as Fierce Deity Link with no problems. This glitch has also been removed from later versions such as the European Nintendo 64 version and the GameCube Collector's Edition.

The third glitch
The third glitch is the easiest and best way to transform and it works with all versions of Majora's Mask, including the GameCube. It can be done in several locations but the easiest is in Termina Field, just outside the entrance to Great Bay. There's a small pool of water off to the side, (right if facing Clock Town, or left if facing Great Bay) which Link must jump into. Once in the water, he needed to be positioned in a spot where he is floating, but when the Zora Mask was put on, he would stand. Once that had been achieved, as Normal Link pressing A to dive and shortly after doing so pressing the C-Button with the Zora Mask equipped, would make you transform like normal. After this by rapidly pressing B and at the same time trying to take off the Zora Mask, the B button would go blank if done correctly. By holding A and swimming in the direction of land, Link should remain stationary and after a few seconds, roll, then stand up normally. Once this had been achieved, anything could be equipped to the C-Button that the Zora Mask was assigned to.

Trapped
Fierce Deity Link is so large (much taller than most characters), that when he enters some buildings, he can not exit, and becomes trapped. A prime example of this is the Stock Pot Inn.

Trivia

 * The discs of light shot from the tip of Fierce Deity Link's sword when a foe is targeted are similar to the Sword Beam, a move absent from Ocarina of Time, The Wind Waker, and Twilight Princess.