The bosses name is sephiroth. He is from fianal fantacy. After you have unloked The World That Never Was(a world) you go back to Hollow Bastion(Or Radiant Garden) you go to the great maw where you fought 1000 heartless and go across facing the evil castle. go across there and you will find a path leadiing to a giant circle and at the very end of the circle there should be a guy with long grey hair(he is sepiroth)WARNING!!:Make sure you are ready for him he is the toughest boss in the game. He has the most HP out of all the bosses he has 13 bars 3,000 HP he is very tough but if you do manage to beat him well done. seperoth will now tell you to go get cloud you will find him in the market place talk to him say yes i know were sepiroth is and you have beaten the toughest boss. then clouds friend will come and she will give you a reward it is..................the last keyblade of your collection it is the Fenir and it is the strongest keyblade of all of them well done!!!!!!!!!!!!!!