Boolossus

Boolossus (also known as the Jumbo Ghost) is a huge Boo and one of the portrait ghosts in the game Luigi's Mansion. The giant ghost is actually composed of fifteen regular Boos who combined their entities to create the powerful Boolossus. Boolossus attempted to capture Luigi on behalf of his leader, King Boo (who in turn was working for the thought to be dead Bowser). Unlike other Boos, Boolossus has glowing yellow eyes.

Luigi battled Boolossus on the balcony of Luigi's Mansion. To defeat the massive 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 a Ice Elemental Ghost from the ice sculpture and began spraying frost at the Boos, freezing the ghosts. After that, Luigi sucked up each Boo, one by one, into his Poltergust 3000. After some time, Boolossus reformed with a smaller number of Boos and resumed his fight against the green-clad hero. Ultimately, Luigi kept popping the big Boo and freezing the small Boos, and eventually defeated the monster. With another portrait ghost defeated, Luigi continued his search for his brother Mario.

A character similar to Boolosus appeared in Paper Mario: The Thousand Year Door; it's name was Atomic Boo.