Broken Prophecy AU
In which the Transcendence occurs during Wierdmageddon; it is, in effect, what stops it.
The prophecy came true, but not in the way anyone expected. In canon, perhaps it was Stan's refusal to complete the wheel that saved all their lives. In this AU, he did not refuse to take Ford's hand and that is how they destroyed Bill and shoved Wierdmageddon back through the rift. However, in the process of doing so, they also destroyed the boundary keeping the magic within Gravity Falls.
Bill was in the physical plane at the time, so at his demise the energy keeping him alive was released. It created a massive flood of demonic energy in a very small area. Five out of the ten people of the prophecy picked up on this energy, inadvertently absorbed it, and to everyone else, vanished, presumed dead.
And in truth, they did die. But their souls didn't move on. Instead, each one was caught in a web of demonic energy, trapping them in the mindscape. Transforming them into demons.
Story to story, those five demons vary, but most often it's the four Pines and Wendy or Pacifica.
The five stick together at first, defending each other from other demons' attacks, very unlike any other demons, who go at it alone. They have their reason for staying together: they all want the same thing. To get back to the physical plane and show their friends they aren't dead.
To the other demons, they end up with aliases. Dipper and Mabel become Alcor and Mizar. Stan and Ford take on the names Alioth and Alkaid, respectively. As the last of the five varies so much, they will remain unnamed here.
Then the summonings start.
Dipper is most often the one pulled away first, but Mabel insisted on coming with him. The summoners are stunned to get two demons instead of one, and it all kinda spirals downhill from there for the cultists who want to destroy the world.
Years later, when the remaining five of the prophecy end up summoning these five new, powerful demons for answers, they discover the five lost friends from so long ago.
And that is the end of the guide. Every story is different, and it would be boring if the AU had to many strict guidelines. Have fun!