How found: First you must collect all 23 other masks in the game, then proceed to the final "dungeon." Give your masks away to the four kids who wear the boss masks and play through their mini-dungeons. After you've completed all four, talk to the kid in Majora's Mask and he'll give you the Fierce Diety's Mask before the final battle. It can only be used in boss rooms.

    How used: The fourth transformation mask turns Link into a bad mofo version of himself, complete with two-handed sword and power to spare. The only catch is that he can't use standard Link items such as the bow, but you'll get over that quickly.