Boolossus

"That scaredy-cat Luigi has made it this far... Oh, we're so frightened! I guess it's our turn, then... All right, Boos, let's assume our real form... But first, how about giving him a little scare!"

- Boolossus

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.

History
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, he makes it to the Balcony. If he has not captured 20 Boos, however, he will be blocked in the hallway leading there by a particularly-transparent Boo, referring to itself as "the mysterious power of the Boos," which will then whisk him back to the Foyer. However, if he has captured the required amount, he can go through. While Luigi is there, the Boo Radar on the Game Boy Horror starts flashing intensely, indicating a powerful Boo presence. To the left, fifteen Boos are circling each other. Once Luigi approaches, they taunt him, followed by beginning to circle around him. They then whisk him away to an ethereal version of the Balcony, where the Boos merge into Boolossus and swoop down upon him.

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.

Trivia

 * Boolossus makes an appearance in the Luigi's Mansion battle course of Mario Kart: Double Dash!! and in the race course Luigi's Mansion in Mario Kart DS as a painting on the wall.
 * Boolossus appears in the Nintendo UNO cards on the "Draw 4" card.
 * In the European version's Hidden Mansion, Luigi rides the Poltergust 3000 much like a bumper car during the Boolossus battle. This provides faster transportation, but also less traction.
 * Boolossus is the only Portrait Ghost to not have HP.
 * A Big Boo assembled from multiple regular Boos similar to Boolossus appears in Luigi's Mansion: Dark Moon, except this one is joined together by 10 Boos instead of 15. This Boo appears in the Treacherous Mansion mission Ambush Maneuver inside the Train Exhibit. In fact, this Big Boo has the same Japanese name as Boolossus.