The Fierce Diety mask transforms Link into an 'onigami' or 'kishin'; a destructive god of Buddhist mythology (pretty out of place in the heavily Celtic world of Zelda). I didn't realize this until studying for this fanart, but 'Oni Link' is enormous! Almost twice the height of a normal Hylian/Terminian.
I wanted to make some fanart to try and sell prints at a local anime convention. Hopefully there are some fans of Zelda or Majora's Mask there.
Oni Link is property of Nintendo.