Fierce Deity's Mask
The Fierce Deity's Mask is the final of the twenty four Masks from Majora's Mask. The Fierce Deity's Mask gives Link the ability to turn into Fierce Deity Link, an omnipotent being that wields the most powerful blade in the game (rivaled only by the Great Fairy's Sword). The Fierce Deity's Mask is the most powerful in the game and may only be used in boss rooms.
Obtaining the Fierce Deity's Mask
After Link has collected all the other masks in the game and arrives Inside the Moon, he sees four Moon Children, each represented by a dungeon boss, running around a tree. While Link completes the four mini-dungeons by talking to each of the running Moon Children, he gives them some of his own non-transformation masks. Once twenty masks have been given to the Moon Children and the four mini-dungeons are cleared by finding the respective Moon Child, another Moon Child representing Majora's Mask gives Link the Fierce Deity's Mask. At this time, it is one of the only masks remaining in Link's inventory. After defeating Majora and saving at the end of the game, the Fierce Deity's Mask becomes a permanent addition to the inventory, and all previously traded masks are once again at Link's disposal.