Link obtains the Fierce Deity Mask after he trades the twenty non-transforming masks to the four Moon ChildrenInside the Moon. The final child sitting by the tree will give Link the Fierce Deity Mask after noticing all his other masks are gone. Wearing this mask enables Link to transform into the Fierce Deity only during boss battles.
In Majora's Mask 3D, Link can also transform into the Fierce Deity inside the two Fishing Holes. If he attempts to leave when in Fierce Deity form, the Fishing Hole Man will keep the young hero from doing so, wondering how he got in there in the first place.
In the Fierce Deity form, Link gains greater strength and range in his attacks as well as certain new abilities. Because the Fierce Deity wields a large, double-handed, helix-edged Sword, he cannot use a Shield, although this disadvantage is compensated for by the Sword's superior power. Link can also use the Sword to release Sword Beams when targeting an enemy.
In the Majora's Mask manga, Link receives the Fierce Deity's Mask from Majora and becomes Fierce Deity Link during his fight against it. In a flashback of Majora, it is revealed that while as a dragon, a mysterious man bearing a strong resemblance to the Fierce Deity in both appearance and clothing, had visited Majora. The man played music for the dragon, who danced for three days and three nights before dying. The man then carved the Majora's Mask from the creature's hide, hoping to seal its power. It is hinted in the story that the stranger is in fact a god, 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 Hyrule Warriors, Fierce Deity Link appears as part of Young Link's Mask weapon type moveset. During Focus Spirit, the youth can temporarily transform into the Fierce Deity to perform powerful attacks before reverting to his normal self, and includes all other bonuses characteristic to Focus Spirit. This form of Focus Spirit lasts unusually longer than any other Warrior. While in Fierce Deity form, Young Link is able to shoot Sword Beams with most of his swings, but generally shares all of his original form's moves. He can also use the Ocarina of Time to convert his Special Attack Gauge into more Magic to refill his Magic Meter, thus extending his time in this form. Young Link's Focus Attack features him slicing the Moon in half, damaging enemies around.
Fierce Deity Link also appears as an alternate costume for Link in the Majora's Mask DLC Pack.
Fierce Deity Speculation
The Fierce Deity's past remains largely unexplained in the game. Like the other Masks, it is implied to contain a powerful spirit. In the manga, the Fierce Deity corrupts even Link, although Link did possess enough control to remove the Mask at the end of the fight against Majora's Mask.
Because Majora gives Link the Mask, calling it the "true bad guy," it can be theorized that Majora orchestrated the events of the game, intending for both itself and the Fierce Deity to be destroyed. However, the context in which it is given, as a child (Majora) designating Link as the Fierce Deity in a game of tag, it is more likely that Majora was simply toying with Link, or was even delusional.
The term "fierce deity" (鬼神) is used to refer to several deities in the Shingon Buddhist pantheon, such as Bishamonten or the Myō-ō, who are fierce and wrathful toward the negative forces that work against humanity's enlightenment, but are benevolent and supportive of humanity. They are seen as deities that work to defeat evil, a quality which the Fierce Deity's Mask gives Link greater ability to achieve.
↑"Hey. Hey! What's with that scary face, fella? If ya leave here lookin' like that, you'll scare away potential customers and ruin our reputation! ...Wait. Hold up. Where did ya even come from...?" — Swamp Fishing Hole Man (Majora's Mask 3D)