Bill Cipher

Bill Cipher is a very powerful demon, summoned by 'Lil Gideon to steal the combination to Stan's safe, from his mind.

History
Gideon summoned Bill with an incantation from his journel, upon arriving the demon demonstrated its incredible powers by altering the landscape around it, ripping the teeth out of the mouth of a deer and then undoing the act. He made the deal with Gideon, in exchange for helping him, Gideon would help him with some unspecified event that ment a lot to him. He also revaled he had been to Gravity falls before, and implied he had previous dealings with Stan.

Upon checking his Journel Dipper found him described as the "most powerful thing" the writer had encountered, as well as a page covered with crossed out text and blood. Using a spell they followed Bill into Stans mind. There he warned them, and gave them a demonstration of his power by blasting a hole in Dipper and bringing two characters Mable was thinking about to life. Before entering the memories to search for the code.

Following working out Stan's reasoning, Mable found the memory and Soos offered to destroy it, however it was then revealed to be Bill who had taken Soos form. Now with the code he contacted Gideon and tried to tell him it, however before he could finish, Mable shot the memory into another one of Stans, in which he was standing over a bottomless pit. As such Gideon called off the deal.

Overcome with rage, he trapped them all in a seperate realm and forced them to endure there worse nightmares, before finally planning to kill them. However they were saved by Dipper, who had learned that it was possible to make things come real in the dream universe by thinking them. Using this they battle Bill, and came close to defeating him. However sick of the fight, he stopped it and once again rendered them powerless, now in a white void. However because he found them persistant he decided not kill them, and instead left, but before he did, he warned them of the coming darkness that would destroy life as they knew it. Then he cheerfully left.