Fierce Deity Link

Fierce Deity Link is a playable character in Majora's Mask. After obtaining all masks in the game and trading them to the Skull Kid on The Moon, the strange creature will reward Link with the Fierce Deity Mask, an all-powerful item that can only be used in boss rooms. Consequently, Fierce Deity Link, the deity that link becomes when dawning this mask, can defeat all bosses in the game quite easily using the power of the Double Helix Sword and sword beams but can only exist in boss rooms.