That, my friends is Oni Link.
He may look like adult Link from Ocarina of Time, but I assure you that he's not. Only after collecting all of the other 23 masks will you find this one. There's just one little problem with this mask...
You can only use it when fighting the end boss! I know, it sucks!
Oni Link is huge and he wields a massive double-bladed sword that's as big as him! The sword that he uses can shoot bolts of energy (the blade beam attack that was mysteriously missing from Ocarina of Time) from the tip. There's not much else I know about him since I haven't played the game yet. Sorry! Here are some screen-shots of Oni Link kicking butt against the final boss!