Known as the Jumbo Ghost, it is a Big Boo comprised of the fusion of many smaller normal Boos.
Unlike normal Boos, Boolossus is much larger, has yellow eyes and the area around it's eyes is darkened.
Boolossus' was captured by Professor Elvin Gadd prior to the events of Luigi's Mansion. In retaliation, King Boo in rage attacked E. Gadd's lab to free Boolossus, the rest of the Portrait Ghosts and then created the Mansion, kickstarting the events of the game.
