Dark King Boo, sometimes called King Dark Boo, is a dark version of King Boo made in Ludwig's laboratory. Ludwig had the idea to make a dark version of him after he saw a Dark Boo enter his lab.
He is going to appear in New Super Mario Sunshine as a boss. He uses Dark Boos and blue flames to attack you, and eventually he will try to follow you. Face the opposite direction and then ground pound when he's close to you; do this three times and he will die.