He's a secret boss battle when you return back to Hollow Bastion. Go back to where you fought the 1000 Heartless and you'll find him there. Be forwarned... he is arguably the hardest boss in the game. He can hit you with a move that allows him to slash you 15 times in a row without you blocking it, summon fire tornados that suck you in, throw Shadow Mines all over the battlefield, hit you with an attack that drains your health down to 1 and your magic down to 0, teleport all over the place, and rain down meteor's on the field. Oh yeah, his combos do sick damage and pretty much kill you if you don't have certain techniques enabled, and his life is 15 health bars long. You're sure to have fun with that!