Henchmaniacs

Bill's henchmaniacs are a group of interdimensional demon criminals who reside in the Nightmare Realm. They all serve under Bill Cipher, a fellow demon who brought them together and helped release them onto Earth. The demons serve as antagonists during the Gravity Falls finale event "Weirdmageddon".

Role in the series
Before the beginning of the series, the demons all indulged in various criminal activities across their respective dimensions before they eventually came to reside in a crawl space known as the Nightmare Realm. When a demon named Bill Cipher came into the dimension, they were all united under him and after Bill took it over, with Bill promising them that he would use the realm's connection to the Mindscape to enable them access to Earth. The demon's sillhouettes are also seen in the episode "A Tale of Two Stans" when Bill is communicating with them through the Dreamscape.

In "Weirdmageddon, Part 1", Bill finally brings the demons to Gravity Falls, Oregon and they prepare to cause havoc. When Ford Pines attempts to shoot Bill back into the rift with his quantum destabilizer, he misses and is captured by Bill who turns him into a golden statue as the demons watch. Avenging Ford, Dipper Pines confronts Bill but is overpowered by him. Bill then tells 8 Ball and Teeth that they can eat Dipper for a snack. However, Dipper is able to get away. Bill then converts a random car into a sleek getaway ride for himself and his other friends. The car flies away to the Fearamid. Later in the episode, the demons party at Bill's Fearamid using the townsfolk as entertainment, and witness Bill vaporize Time Baby and the Time Police after the group arrives trying to arrest the demons.

In "Weirdmageddon 2: Escape From Reality," Bill's friends attempt to leave Gravity Falls to spread their chaos across the world, only to collide with an invisible barrier. Bill decides to inquire Ford how to get out, since he is the only one who know how.

In "Weirdmageddon 3: Take Back The Falls," the rebel group from the Mystery Shack encroach upon the Fearamid using the Mystery Shack-converted Shacktron. Bill sends his henchmaniacs to fight off the mech, growing to colossal size as they do. Eventually, the Shacktron defeats all the demons and allows the rebels to enter the Fearamid. However, after Bill destroys the Shacktron, the demons corner the remaining rebels. Following Bill's defeat, the demons are sucked back into the nightmare realm.

Members
8 Ball=A green lanky demon covered in shackles with magic 8-balls for eyes. It along with Teeth attempted to eat Dipper, but failed to do so. It was partially petrified by an Eye-Bat while fighting Shacktron.
 * -|Amorphous Shape= A demon composed of many colorful squares. It was the only demon that did not participate in the fight against Shacktron.
 * -|Hectorgon=A red hexagon-shaped demon with a tie, hat, mouth and mustache.
 * -|Keyhole=A diminutive turquoise demon with a large forehead containing a keyhole-shaped hole.
 * -|Kryptos=A square-shaped demon with one eye similar to Bill.
 * -|Pyronica=A feminine humanoid demon surrounded in pink flames.
 * -|Paci-Fire= A dark grey infant-like demon with two large horns, a cross marking and a pacifier on its stomach, which conceals a second face.
 * -|Teeth= A demon that is a set of teeth with stout arms and legs. It along with 8-Ball attempted to eat Dipper but failed.
 * -|Xanthar=A large, cubic demon resembling a loaf of bread.

Trivia

 * In the altered opening sequence for "Weirdmagedden", the demons take the place of the series' main characters seen in the opening.