Boolossus

Boolossus, the Jumbo Ghost is the third major boss in the game Luigi's Mansion, found in Area Three. He is made up of fifteen normal Boo ghosts that combine together on the balcony upon Luigi's arrival.

After he has been defeated, the Diamond Key is given to Luigi. Approaching the Diamond Door on the Balcony will cause a power outage in the mansion, causing many ghosts to attack in the darkness, regardless of the room. Boolossus is seen as a large Boo. His only attack is bouncing. When he splits up, the boos will try to ram into Luigi, though most will flee.

Defeat
Luigi must pop Boolossus with the horn of the nearby frozen unicorn statues. He can then obtain an Ice Elemental Ghost from the statue. Luigi must then freeze the Boos, and after that he must suck them up.