Shadow King Boo

Shadow King Boo is King Boo possessed by the Shadow Queen. His attacks are unchanged except when he is damaged enough, he absorbs other spirits, copying their abilities. After being defeated, King Boo escapes and Luigi is granted the Key to the eighth Mansion and King Boo's Crown which let's Luigi play the minigame "Revenge of King Boo".

Trivia

 * This is the second game in which King Boo is not the final boss, the first being Super Mario Sunshine.