Ug is the ninth boss encountered in Luigi's Mansion 3. He is a caveman ghost who possesses a T-Rex fossil in the Unnatural History Museum of The Last Resort. He guards the portrait containing Blue Toad as well as the Basement 2 elevator button, though unlike the others he does not directly possess the button; it is instead being held by the Toad he is guarding.
Ug is a muscular yellow ghost bearing an appearance similar to that of a caveman. He has a slightly hunched posture, red eyes and mouth, and black hair.
Ug's presence is hinted at when Luigi enters the main room of the Unnatural History Museum during a thunderstorm; with each flash of lightning, the T-Rex's skull turns to look at his current position. When Luigi finds Toad's portrait and begins freeing him with the Dark-Light Device, the T-Rex fossil comes to life and catches the painting in its jaws. As Luigi runs for cover, the skeleton swallows the painting, causing it to be absorbed into the orb of light within its rib cage; with his prisoner secured, Ug turns his attention to Luigi.
After taking enough damage, the T-Rex fossil collapses into a pile of bones. Ug finally reveals himself as he pulls himself from the rubble, shaking off his daze before pulling a bone from the ruined skeleton to use as a club. He promptly demonstrates his strength by slamming the bone into the floor, creating a shockwave that scatters the rest of the T-Rex skeleton; however, this also reveals his weakness when he struggles to pull the bone back out.
The first phase deals with the T-Rex fossil that Ug has possessed. The player has to pick up one of the eggs dropped from the pterosaur exhibits dangling from the ceiling, waiting for the T-Rex to lower its head before shooting the egg into its mouth. As Ug's vessel tries to crush the egg in its jaws, Luigi can pick up the other egg lying around and shoot it at the skeleton's exposed chest. After doing this once, the T-Rex will roar and destroy one of the exhibits, leaving only one egg available as ammunition. Therefore, Luigi must trick the T-Rex fossil into biting Gooigi for its chest to be vulnerable; he will gum up the dinosaur's jaws long enough for Luigi to shoot another egg at its ribs.
After taking two hits, the T-Rex will break the pole holding it in place, also shattering the remaining pterosaur skeleton as it becomes fully mobile. At this point, it will chase Luigi around the room, attacking with stomps, bites, waves of sonic energy from its jaws, and tail swipes that hit in a full ring around it. To damage the skeletal dinosaur, Luigi needs to pick up bits of the other exhibits as they're destroyed by Ug's rampage and fire them into the T-Rex's chest when it rears up; if they run out of fossil parts, they can use debris dropped from above when the T-Rex stomps the ground. Each hit destroys some of the fossil's ribs. After four hits, the skeleton crumbles and Ug is forced into the open.
Ug attacks Luigi with various sweeps and smashes of his bone club. When he uses his shockwave attack, which he telegraphs by raising his club over his head as it glows red, Luigi must dodge the wave with a Burst, then flash Ug with the Strobulb while he is struggling to pull the bone out of the floor. He can then begin sucking him into the Poltergust G-00.
Upon being defeated, Ug will be sucked into the vacuum, throwing his club into the air amid his panicked flailing. His head gets stuck in the nozzle, only to be forced in after his own club lands on it, allowing Luigi to release the Blue Toad's portrait and finish freeing him. After emerging and being comforted by Luigi, the Blue Toad reveals that he has the Basement 2 button and gives it to his savior before being escorted to the elevator.
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