Alcor D. Cipher
In this story, Bill is more of a brother to Dipper, along with his brother Will.
An incident in the past happened, Dipper had always known because Bill don't feel the need to keep it a secret.
Dipper had always been a Demon since birth but some human, he have a strange birth mark which seems to contain some big secret or something, and now Dipper is on the run with his brothers and get revenge on humanity.
This is a transcendence AU in my version, and Mabel might not have powers.
Also, images are not mine, even the cover.