In the latest issue of US magazine Game Informer, Zelda producer Eiji Aonuma talked about the Fierce Deity Mask.
In The Legend of Zelda: Majora's Mask, many of the masks were forged to contain the memories of those who had passed away. Aonuma suggests that "becoming them is actually really painful because it's like hosting a really powerful spirit that's coming into you."
The story behind the primary masks in the game were explained, however it's left some questions about the Fierce Deity Mask - just whose soul is in there? Aonuma replied, "The best way to think about it is that the memories of all the people of Termina are inside of the Fierce Deity Mask."
What are your theories behind the mask?