The Sword of Light wasn't a keyblade by the sounds of it, but it could dispell darkness if the legends were true. ..Not that it mattered, corrupted as it was now by the very power it was meant to vanquish.

"I think I understand," he said slowly. A thousand thoughts overlapped and left him dizzy, but they spiralled around a central truth. He was closer than ever to understanding what he needed to do. "For Xion to be whole again, her Heartless has to be defeated by a keyblade? And the keyblade is a weapon that dispels darkness? ..I wonder if that isn't the only way."