Boolossus

Boolossus, the Jumbo Ghost, is a huge Atomic Boo and one of the portrait ghosts in the game Luigi's Mansion. This giant ghost is actually composed of fifteen regular Boos who combined their entities to create the powerful Boolossus. Although these Boos are normally timid and shy, when they form into Boolossus, all their fears disappear. The massive Boo is a servant of King Boo, leader of the Boos, and is the guardian of the third area of Luigi's Mansion. Unlike other Boos, Boolossus has glowing yellow eyes.

Before the events of Luigi's Mansion, Boolossus was actually captured by Professor E. Gadd during the professor's travels around the world. E. Gadd used his Portrificationizer to transform the gigantic ghost into a painting and placed the ghoulish piece of art in his gallery of other portrait ghosts. Unfortunately, the capture of Boolossus greatly angered King Boo. As such, 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 turned the Boolossus painting back into their gigantic Boo comrade. 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 magically created a mansion out of thin air, right next to E. Gadd's laboratory in Boo Woods. Boolossus and King Boo, along with all the other Boos and portrait ghosts, decided to inhabit the mansion and haunt its halls. Ultimately, it would be this mansion that Luigi would win in a contest he never entered. And it would be this mansion where Luigi's skills would be put to the test as the green-clad hero attempted to save his brother Mario from the clutches of King Boo. During the events of Luigi's Mansion, Boolossus, along with the other Boos (including King Boo) decided to take a rest in a secret area under the Storage Room. When Luigi activated a hidden door in the Storage Room while searching for his brother, the Boos sprang out, enraged after being awakened. However, the Boos noticed E. Gadd's Poltergust 3000, a ghost-catching vacuum device, on Luigi's back. As such, the Boos, including Boolossus and King Boo, decided to run off and hide in the mansion, instead of challenge the novice ghost hunter.

Eventually, as Luigi made more and more progress through the mansion, Boolossus attempted to capture the green-clad plumber on behalf of his leader, King Boo. After Luigi reached the Balcony on the third floor, Boolossus decided to make his move. While on the Balcony, Luigi noticed the Boo Radar on his Game Boy Horror started flashing like crazy, indicating a powerful Boo presence. Soon, the fifteen Boos that made up Boolossus appeared before Luigi, circling and taunting the novice ghost hunter. After having their fun, the Boos decided to get serious. They formed into the gigantic Boolossus and whisked Luigi away to the balcony arena, starting the battle to capture the green-clad hero and turn him into a portrait, just like his brother Mario. During the battle, Boolossus attempted to slam into Luigi and cause massive damage. To defeat the giant Boo, Luigi used his Poltergust 3000 to pull Boolossus into one of the two conveniently placed unicorn ice sculptures located on the balcony. When Boolossus hit the sharp horn of the statue, he popped and separated into the fifteen normal Boos. Luigi then absorbed an Ice Elemental Ghost from the ice sculpture and began spraying frost at the Boos, freezing the ghosts. After that, Luigi sucked up each frozen Boo, one by one, into his Poltergust 3000. Eventually, Boolossus reformed with a smaller number of Boos. Although smaller in size than before, Boolossus resumed the fight against the green-clad hero. However, Luigi kept popping the big Boo and freezing the small Boos, slowly draining Boolossus's power. Unfortunately, as Luigi caught more and more Boos, the other ghosts would become more and more wary of the Poltergust's ice blasts. As such, Luigi had to pull these careful Boos closer to his position with the vacuum device then shoot them with a sudden stream of icy mist. Ultimately, after a long and difficult battle, Luigi froze and captured each of the fifteen Boos that made up Boolossus.

After capturing Boolossus, Luigi acquired the key to the West Wing of the mansion. Luigi also noticed that his Game Boy Horror indicated that the plumber had captured fifteen more of the fifty released Boos, meaning there were only a few more Boos hiding in the mansion. After the battle, the ghost hunter Luigi visited Professor E. Gadd's lab, and returned Boolossus to his portrait prison. Luigi also took a break from ghost hunting for some time and had dinner with Professor E. Gadd in the laboratory.

Trivia

 * Another instance of many Boos composing a giant Boo occurred in Paper Mario: The Thousand-Year Door. An optional boss battle was with an Atomic Boo. However, instead of being made up of fifteen Boos, this Atomic Boo was made up of around 200 Boos (although he was strangely smaller than Boolossus. This could be because the 50 Boos in Luigi's Mansion are stronger than the average Boo).
 * Boolossus made an appearance in the Luigi's Mansion Battle Field of Mario Kart: Double Dash!! as a painting on the wall. Other portrait ghosts appeared as well.
 * Boolossus appears in the Mario UNO cards in the "Draw 4" card.
 * In the PAL Hidden Mansion, Luigi rides around sitting on top of the Poltergust 3000 during this battle, which makes him move a lot faster as if he were on ice. He is also only able to turn using the C-stick when riding the vacuum.
 * In Mario Kart DS, he appeared in the course Luigi's Mansion in one of the pictures.