Boolossus

The Boolossus is a giant-sized Boo that is actually an amalgamation of fifteen smaller Boos fused into a single form. This creature appears as a boss in Luigi's Mansion on the Nintendo Gamecube.

History
Before the events of Luigi's Mansion, the Boolossus was captured by Professor E. Gadd during the professor's travels around the world. Gadd used his Portrificationizer to transform the Boolossus into a painting and placed the painting in his gallery of other portrait ghosts. However, the capture of the Boolossus greatly angered King Boo, who retaliated by leading his Boo minions in an assault on Professor Gadd's laboratory. The Boos reversed the setting on the Portrificationizer and reverted Boolossus' state. King Boo then ordered that all the other portrait ghosts that the Professor caught should be freed as well. Eventually, after all the portrait ghosts were released, King Boo created a mansion, right next to Gadd's laboratory in Boo Woods.

During Luigi's Mansion, Luigi accidentally releases the Boos, the Boolossus among them, from a hidden section beneath the mansion's storage room. Noticing the Poltergust 3000 that Luigi is carrying, the Boos scatter throughout the mansion and Luigi must explore further into the house to find and capture them. The Boolossus will hide away on the mansion's balcony where Luigi cannot go until he has captured at least twenty Boos. Once he has captured the required amount, he can go through. While Luigi is on the balcony, he spots fifteen Boos that begin circling around him before coming together to form the Boolossus, beginning the boss battle.

During the battle, the Boolossus attempts to slam into Luigi. To defeat it, Luigi must use his Poltergust 3000 to pull the Boolossus into one of the two stationary unicorn ice sculptures located on the balcony. When Boolossus touches the statue's sharp horn, it pops like a balloon, separating into the fifteen normal Boos. Luigi must then absorb an Ice Elemental Ghost from the ice sculpture and spray frost at the Boos, freezing them. This enables him to suck them into his Poltergust 3000. Eventually, The Boolossus reforms with a lower number of Boos. It becomes faster every time several of the Boos are sucked up. However, if Luigi keeps popping it and freezing the small Boos, he can slowly drain the Boolossus' power. As more and more Boos are caught, the other Boos become more wary of the Poltergust's ice blasts and flee when the ice blasts get too close to them.

After capturing the Boolossus, Luigi acquires the key to the West Wing of the mansion. Defeating the boss counts as capturing a total of fifteen of the fifty released Boos. After the battle, Luigi visits Professor Gadd's lab and turns the Boolossus into a painting.