Fierce Deity

Fierce Deity (鬼神) is the presumed name of a character from The Legend of Zelda: Majora's Mask. He is the spirit that is assumed to inhabit the Fierce Deity's Mask. There are very few facts truly known about the Fierce Deity.

When Link dons the Goron Mask, Zora Mask, or Deku Mask, he transforms into a form similar to Darmani, Mikau, or what is presumed to be the Deku Butler's Son, respectively. Due to this, it is believed that the spirit inhabiting the Fierce Deity’s Mask is that of a great god. When Link uses any particular transformation mask, he takes the shape of what the spirit inhabiting the mask looked like during its lifetime, with a few characteristics altered to look more like Link. Using this knowledge, it can be surmised that the Fierce Deity was a tall warrior god, who wielded the a sword which was capable of shooting Sword Beams. Like with the other transformation masks, his exact appearance may be different from the one seen when Link assumes his shape.

It is possible, given that the other transformation masks are named for the race they represent and not the person, that "Fierce Deity" is the name of a race as opposed to an individual, but unlike the other transformation masks, the mask is called the Fierce Deity's Mask (using the possessive of Deity), which may indicate that the mask once belonged to a character known as the Fierce Deity. This may actually be because this Mask is a parallel to Majora's Mask, which is possessed by a spirit hence it's name.

In addition to the Fierce Deity appearing as a transformation of Young Link's in Majora's Mask and the Hyrule Warriors spin-off series, the Fierce Deity is referenced in other titles such as The Legend of Zelda: Tri Force Heroes.

Majora's Mask
The Fierce Diety was apparently an ancient hero from the world of Termina. Its has the power to empower link with the power of the Fierce Deity.

Breath of the Wild
A Fierce Deity set could be obtained in Breath of the Wild using the amiibo Rune and the unreleased Majora's Mask amiibo. These include his iconic clothing, and his sword, and the sword. The lore claims this was the armor of a hero from another world who obtained godlike powers. This is either a reference to young link (who actually wasn't from Termina, or the ancient hero the mask and gear represents). This set includes the Fierce Deity Mask, Fierce Deity Armor, and Fierce Deity Boots. It is also possible to obtain the Fierce Deity Sword as well.

Theories
Though the history of Fierce Deity is never elaborated upon in the game, it is implied, through what is said by the likes of Tatl, and the fact that Majora himself gives Link the god's mask, that the Fierce Deity's Mask was created by a Deity far stronger than Majora, and that Majora could possibly have given the mask to Link in an effort to destroy them both. Fierce Deity's powers may be even darker than that which resides in Majora's Mask, as Majora calls him "the true bad guy", though this may just be a lie or exaggeration on the part of Majora, and that Majora considers itself a hero. This statement is only present in the dubbed version however, and not the original Japanese version.

Another theory is that the Fierce Deity is Link's Terminan counterpart, due to the resemblance between him and Link, as no definite counterpart to Link is seen in-game. Though Link's other alternate forms retain some aspects of his appearance, the transformation masks themselves do not. However, the Fierce Deity's Mask itself shares Link's hat and some of his facial features even when not in use, and while in use, Fierce Deity Link's voice is the same as that of Adult Link from The Legend of Zelda: Ocarina of Time. The description of the mask as being filled with dark power does not affect this theory, since morality is not always consistent between Terminan and Hyrulean counterparts, as seen by the differences between the two incarnations of Koume and Kotake. The Fierce Deity's status as a god would seem to indicate that it is not Link's equivalent, though its power could also be interpreted as a more direct embodiment of Link's defining characteristic of being destined to accomplish legendary feats.

Though there is little evidence to support this theory, it has been suggested by some that when Majora realizes Link is a threat, it seals its spirit into the Fierce Deity's Mask instead of Majora's Mask in order to survive. If Link chooses to wear the mask, he succumbs to a great power, defeating Majora easily. If he chooses not to, he must face a far more difficult battle, but has not given into the possible trickery of Majora. This theory presents the Fierce Deity's Mask as a moral choice to the player; whether or not to use the possibly evil powers of "the true bad guy" to defeat a final boss. Moral choices are rarely present in the Legend of Zelda series, but it would make sense that one of this magnitude would be present in Majora's Mask, a game known for its intricate plot. This would explain why such a powerful weapon is simply gifted to Link, and would maybe reveal a darker, unseen ending to Majora's Mask. By wearing the Fierce Deity's Mask, Link sells his soul much in the same way that the Skull Kid had done with Majora's Mask. Therefore, when Link returns the emptied Majora's Mask to the Happy Mask Salesman, Majora has tricked both the Happy Mask Salesman and the player, and could survive on in the Fierce Deity's Mask. It could also explain why the final boss battle appears to feature incarnations of Majora and may never show the entity's true form. This theory does not explain how Majora would continue to act through Majora's Mask while inhabiting the Fierce Deity's Mask, if it would even be possible for the entity to leave Majora's Mask, and why Majora would give Link such awesome power rather than wield it itself. According to the Hyrule Historia however, the Hero of Time lived long enough to have at least one child which leads to his descendant, the Link in Twilight Princess whom he assisted as the Hero's Shade, showing that he wasn't controlled by any evil entities whatsoever. Of course it is possible that he could have freed himself from its influence at some point following the events of Majora's Mask.

Hyrule Warriors series
The Fierce Deity's mask appears as bronze and gold badges "Fierce Deity's Mask I" and "Fierce Deity's Mask II" which increases a character's Special Attack gauge by one bar.

In the Majora's Mask DLC Pack, the Fierce Deity appears in certain parts of Young Link's Mask moveset such as his weak point attack and his special attack. Young Link also transforms into the Fierce Deity when using Focus Spirit and Focus Spirit Attack. During his Focus Spirit attack the Fierce Deity uses his sword to slice the Moon, summoned by Majora/Skull Kid, in two. The Fierce Deity also appears in Young Link's Victory cutscene. The Fierce Deity also appears as an unlockable costume for Link.

In Hyrule Warriors Legends, the Fierce Deity returns as a transformation for Young Link and as an unlockable costume for Link. There is also a Fairy Clothing option for Companion Fairies called Fierce Deity Facepaint.

The Legend of Zelda: Majora's Mask manga
It is explained that Majora's Mask is carved from the armor of a powerful dragon, Majora. Majora guarded an empty land that was neither moving nor dead. This dragon's armor was sought after, as it was believed that the armor could grant wishes and give power. Many came hoping to claim the armor for their own reason, be them good or evil, but Majora killed them all. One day, a mysterious man visited Majora. This man played music for the dragon, who danced for three days and three nights before dying, supposedly allowing time to move forward. The man then carved a mask from the creature's hide, hoping to seal its power.

Interestingly, the traveler bears a strong resemblance to Fierce Deity, who has darker-than-average skin, and silver hair, and it is hinted in the story that the stranger is in fact a god, although why this being chose to approach and ultimately slay the beast is never explained, other than when he says, "I was drawn to this land by a strange force". Some further evidence pointing to this being the true identity of Fierce Deity, a point is made that the stranger is the first being that is able to overcome the beast of Majora, and that it does so with music that controls the flow of time.

In the Manga, he was referred to as the "Ogre Deity", a too-literal translation of the Japanese name of the mask. Oni usually means ogre or demon, but the use of the 鬼 (oni) character in the compound 鬼神 (kishin, fierce deity) is intended to ascribe the wrathful aspect of oni to the deity (神) thus described.

Super Smash Bros. Brawl
Fierce Deity Link is featured as a sticker. Its sticker gives a +21 bonus to slash attacks, and can only be applied to Link, Toon Link, Zelda, and Ganondorf.

Super Smash Bros. for Nintendo 3DS / Wii U
Fierce Deity Link is featured as an alternate costume for Link. However the costume only alters Link's face into that of the Fierce Deity Mask and the color of his tunic, Hylian Shield, and Master Sword. The Fierce Deity's signature sword is also not present.