Boolossus, the Jumbo Ghost, is an enormous Boo and one of the portrait ghosts in the game Luigi's Mansion. This ghost is composed of fifteen regular Boos who can combine to create Boolossus. Although these Boos are normally timid and shy, when they form into Boolossus, all their fears disappear. The Boo is a servant of King Boo, leader of the Boos, and is the guardian of the third area of Luigi's Mansion. Its name is a pun on the words "Boo" and "colossus" and literally translates to "Giant Boo". Unlike other Boos, Boolossus has glowing yellow eyes.
Before the events of Luigi's Mansion, Boolossus was captured by Professor E. Gadd during the professor's travels around the world. E. Gadd used his Portrificationizer to transform Boolossus into a painting and placed the painting in his gallery of other portrait ghosts. However, the capture of Boolossus greatly angered King Boo. In retaliation, King Boo led his Boo minions in an assault on Professor E. Gadd's laboratory. The Boos reversed the setting on E. Gadd's Portrificationizer and reverted Boolossus's state. King Boo then ordered that all the other portrait ghosts that Professor E. Gadd caught should be freed as well. Eventually, after all the portrait ghosts were released, King Boo created a mansion, right next to E. Gadd's laboratory in Boo Woods.
In Luigi's Mansion, Boolossus, along with the other Boos (including King Boo), rests in a secret area under the Storage Room. When Luigi activates a hidden door in the Storage Room, the Boos spring out. However, when they notice E. Gadd's Poltergust 3000, a ghost-catching vacuum device, on Luigi's back, the Boos decide to run off and hide in the mansion.
Eventually, as Luigi progresses through the mansion, Boolossus attempts to capture Luigi on the third floor Balcony. While Luigi is there, the Boo Radar on the Game Boy Horror starts flashing intensely, indicating a powerful Boo presence. Soon, the fifteen Boos that make up Boolossus appear before Luigi, circling and taunting him. They soon merge into Boolossus and whisk Luigi away to the balcony arena, starting the battle.
During the battle, Boolossus attempts to slam into Luigi. To defeat Boolossus, Luigi must use his Poltergust 3000 to pull Boolossus into one of the two stationary unicorn ice sculptures located on the balcony. When Boolossus touches the statue's sharp horn, it pops, 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, 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 Boolossus's 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 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 E. Gadd's lab and turns Boolossus into a portrait.
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