The Fierce Deity's Mask is an item from LoZ:Majora's Mask.
The Fierce Deity's Mask is an item that will transform its wearer into the powerful, and evil, Fierce Deity. The Deity's power is matched by no one, and will take control of the person who wears its mask. The person will have no control of whatever they do while wearing the mask.
Dark Link found the mask locked up in a chest inside Ganondorf's dungeon. He plans on using it to trick Link.