Xanthar appears as a giant purple demon who walks on all four of its ape-like legs, and has no head. In its place is a light-purple patch much like a loaf of sliced bread. It has small trees growing out of its back and shoulders, and it wears a blue and purple colored party hat with a yellow ball on top. Xanthar's body is shaped like a cube.
Eons before the events of the series Xanthar committed crimes before being called together by Bill Cipher, along with 8 Ball, Amorphous Shape, Hectorgon, Keyhole, Kryptos, Paci-Fire, Pyronica, and Teeth.
In Weirdmageddon Part 1," Bill introduces Xanthar along with the rest of his demon friends to the residents of Gravity Falls. Afterwards, on Main Street, Xanthar and the other demons watch Bill turn Ford into a golden statue and taunt Dipper before burning the journals. Later on, Xanthar is seen playing "Spin the Person" and enjoying the party Bill is having at the Fearamid until Time Baby and the Time Police invaded the Fearamid, but to the demons delight Bill vaporizes Time Baby and his squadron except Blendin. The party then continues as Xanthar and the other demons rejoice over Time Baby's presumed death.
In "Weirdmageddon 2: Escape From Reality," Xanthar and the rest of Bill's minions attempt to leave Gravity Falls and spread chaos across the world, but are stopped by an invisible barrier which makes Bill considers getting Ford 'out of retirement'.
In "Weirdmageddon 3: Take Back The Falls", he along with Bill's minions attacked the Shack-tron during the giant monster fight. But was tossed into the mountains, leaving behind only his party hat. He was sucked back to the nightmare realm following Bill's defeat along with the rest of the nightmares.