Ug is the ninth boss encountered in Luigi's Mansion 3. He is a caveman ghost who possesses a Tyrannosaurus rex skeleton in the Unnatural History Museum of The Last Resort. He guards the portrait containing the blue Toad as well as an elevator button that will grant access to the Boilerworks, 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 mustard-yellow ghost. He has a slightly hunched posture, pupilless red glowing eyes and mouth, a few teeth (with one sticking out) and black-brown hair. He is dressed in an orange fur cloth and wields a bone club.
Ug's presence is hinted at when Luigi enters the main room of the Unnatural History Museum as a rainstorm approaches; with each flash of lightning, the Tyrannosaurus rex skeleton'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 skeleton comes to life and snatches 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 the painting secured, Ug turns his attention to Luigi and starts attacking.
After taking enough damage, the skeleton 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 remains of the skeleton he possessed; however, this also reveals his weakness when he struggles to pull the bone back out.
The first phase requires the player to destroy the Tyrannosaurus rex skeleton Ug is possessing. The player has to pick up one of the eggs dropped from the pterosaur skeletons dangling from the ceiling, waiting for Ug to lower its head before shooting the egg into its mouth. As Ug 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, Ug will roar and destroy one of the exhibits, leaving only one egg available as ammunition. Therefore, Luigi must trick Ug into gnawing on Gooigi to make the skeleton's chest vulnerable; he will gum up the skeleton's jaws long enough for Luigi to shoot another egg at its chest.
After taking two hits, Ug will break the pole holding the skeleton 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 skeleton, Luigi needs to pick up bits of the other exhibits as they are destroyed by Ug's rampage and fire them into the skeleton's chest when it rears up; if resources are scarce, he can use debris dropped from above when Ug stomps the ground. After four hits, the skeleton crumbles and Ug reveals himself.
Ug attacks Luigi with various sweeps and smashes of his bone club. He can also use a shockwave attack, which he telegraphs by raising his club over his head as it glows red. Luigi must dodge the wave with a Burst. If he gets hit by the shockwave, he will become dazed and cannot move for a few seconds. He must 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 Poltergust, 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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