8 Ball

8 Ball is a character in Gravity Falls, appearing in the episode "Weirdmageddon Part 1." He is a flesh eating dream demon and an old friend of Bill Cipher and part of the Gang of Interdimensional Criminals and Nightmares. He, along with Bill and the rest of the gang, was trapped in a decaying dimension, but was finally set free when the rift between the dimensions cracked during the prophesied Weirdmageddon.

Season 2
In "Weirdmageddon Part 1," when Bill defeats Ford and burns the Journals, leaving a distraught Dipper behind, he tells 8 Ball and Teeth to be free on eating Dipper, if they want, before heading to the party organized in the Pyramid.

Precisely when the Time Police crashes the party and the Time Baby is vaporized, 8 Ball and Teeth arrive to the party and warn Bill that Dipper is still roaming free, and might just try to free Mabel from her bubble. However, Bill isn't to worried, as he has Gideon on the case, so the party carries on.

Trivia

 * The skeleton shown in the theme song shares various similarities to 8 Ball.