How do you KILL the black knight in the game sonic and the black knight

You don't, at least not until the very end. You have to kill the 3 Knights of the Round Table. By the time you figure this out you should have already killed 2 knights and all there is left is the one that looks like Amy. Defeat her, visit the Lady of the Lake. She'll combine the three swords you'll get with Caliburn and you will be able to destroy immortality. Awsome isn't it, lol.