Xanthar

Xanthar, also known as The Being Whose Name Must Never Be Said, is an interdimensional criminal from an unknown dimension who appears during the events of Weirdmageddon.

Background
Eons before the events of the series, Xanthar indulged in criminal activities before being called to join Bill Cipher, along with 8 Ball, Amorphous Shape, Hectorgon, Keyhole, Kryptos, Paci-Fire, Pyronica, and Teeth.[3]

Season 2
In "Weirdmageddon Part 1," during the cold open, Bill introduces Xanthar with the rest of his 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 in the episode, Xanthar is seen playing "Spin the Person" and enjoying the "V.I.P." party Bill is having at the Fearamid until Time Baby and the Time Police crash the party. To the beings' delight, Bill vaporizes Time Baby and his squadron. 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 their chaos across the world, but are stopped by an invisible barrier. They are later subjected to Bill's rage over their inability to leave.

In "Weirdmageddon 3: Take Back The Falls", Xanthar participated in the fight against the Shacktron, during which it rammed the gigantic robot, only for the robot to toss it into the mountains, leaving behind only its party hat. It was sucked back into the nightmare realm along with its fellow comrades following Bill's defeat.