Bill Cipher (voiced by Alex Hirsch) is a powerful interdimensional dream demon and the main antagonist of the series.[4][5] He is a one-eyed yellow triangle that resembles an Eye of Providence and wears a top hat and a bow tie. Living in the Nightmare Realm, an interdimensional area lacking consistent laws of physics, Bill has the ability to make deals with and possess others. He first appears physically in "Dreamscaperers", where Li'l Gideon summons him to steal Grunkle Stan's deed to the Mystery Shack. In "Sock Opera", Bill tricks Dipper into making a deal with him, briefly possessing his body until Mabel forces him out. In "Dipper and Mabel vs. the Future", Bill possesses Blendin Blandin to release a dimensional rift, which allows Bill to escape the Nightmare Realm. In "Weirdmageddon", Bill takes on a physical form, unleashes various monsters from the Nightmare Realm, and takes over Gravity Falls. However, he realizes that a force field prevents him from escaping Gravity Falls to conquer the rest of the universe. Bill fails to destroy the force field and is apparently destroyed for good by Stan. However, his last words, when played in reverse, are revealed to be "A-X-O-L-O-T-L MY TIME HAS COME TO BURN, I INVOKE THE ANCIENT POWER THAT I MAY RETURN!" In the book Dipper and Mabel and the Curse of the Time Pirates' Treasure!: Select Your Own Choose-Venture, there is a coded link that leads to a hidden interaction between Dipper and Mabel and the Axolotl, in which Dipper asks about Bill Cipher. The Axolotl responds with the following poem: "Sixty degrees that come in threes. Watches from within birch trees. Saw his own dimension burn. Misses home and cannot return. Says he's happy. He's a liar. Blame the arson for the fire. If he wants to shirk the blame, He'll have to invoke my name. One way to absolve his crime. A different form, a different time."
The crime referenced in the poem is further detailed in the Journal 3 and The Book of Bill which state that he caused a catastrophe that resulted in the annihilation of his home dimension Euclydia, known throughout the multiverse as "The Euclidean Massacre." Although Bill pejoratively describes the Flatland-like Euclydia as "flat minds in a flat world with flat dreams," The Book of Bill references multiple times the immense guilt he feels, and implies the Euclidean Massacre may not have been intentional. The Book of Bill reveals that because of his final words, Bill is sent to a "tank outside of space" where a deity known as the Axolotl resides.[6] Bill is spared by the Axolotl and sent to the Theraprism, an interdimensional mental hospital, with the book itself revealed to be created by Bill during therapeutic arts and crafts sessions.[6]
