8 Ball is an interdimensional criminal from an unknown dimension who appears during the events of Weirdmageddon.
Eons before the events of the series 8 Ball indulged in criminal activities before being called together by Bill Cipher, along with Amorphous Shape, Hectorgon, Keyhole, Kryptos, Paci-Fire, Teeth, Pyronica, and Xanthar.
In "The Last Mabelcorn," after Ford receives Bill's warning that Weirdmageddon is coming, 8 Ball's arms are briefly seen on an image of a wheel with symbols. Near the end of the episode, in a flashback, when Ford confronts Bill for lying to him, 8 Ball's laughing silhouette is seen in the representation of another dimension projected in the Mindscape.
In "Weirdmageddon Part 1," during the cold open, Bill introduces 8 Ball along with the rest of his friends to the residents of Gravity Falls. Afterwards, on Main Street, 8 Ball and the others watch Bill turn Ford into a golden statue, taunt Dipper and burn the journals. Bill then tells 8 Ball and Teeth to eat Dipper as a reward. However, Dipper manages to get away from them. Later in the episode, after Time Baby and the Time Police are vaporized, 8 Ball and Teeth approach Bill to warn him that Dipper is still roaming free, and might try to free Mabel from her bubble. However, Bill isn't worried, as he has Gideon on the case, so the party carries on.
In "Weirdmageddon 2: Escape From Reality," 8 Ball and the rest of Bill's friends attempt to leave Gravity Falls to spread their chaos across the world, only to collide with an invisible barrier. They are later subjected to Bill's rage over their inability to leave.
In "Weirdmageddon 3: Take Back The Falls," 8 Ball participates in the battle against the Shacktron, during which the being is hit by one of the Eye Bats' beams when Wendy hijacks it, turning most of 8 Ball's head into stone. Following Bill's defeat, 8 Ball is sucked back into the nightmare realm along with its comrades.