The Fierce Deity's Mask is the 4th Transformation Mask Link gets and is the final mask obtained in "Majora's mask". Donning the mask transforms Link into a extremely powerful, perhaps even godlike warrior with abilities and powers beyond any Link had ever known. In fact, it's been said that the power within the mask was so frighteningly great that even courageous Link was afraid to put the mask back on for fear of what destruction he could cause. In that sense, though the Fierce Deity's Mask was used to vanquish the evil of Majora's Mask, the fact that the mask would become an object of fear for Link makes the Fierce Deity's Mask no less malicious than Majora's Mask itself. There is belief by some that the Fierce Deity's Mask is also an evil power and that the only reason it had a positive effect in the storyline was because it was worn by a righteous being. But if this is true, then that would also mean that Majora's Mask could also have positive effects if worn by the right person. But this theory will never be tested, since Majora's Mask was destroyed and the Fierce Deity's Mask was discarded (?) by Link after that battle.