Spoilers for the finale (if that wasn't obvious)


Bill acted as fast as he could considering he could feel himself literally falling apart. He invoked the power of rebirth. He would not fall today! No!

As he burst apart, courtesy of Stanley Pines, he cursed the Pines. He put everything he had left into ensuring that he'd be reborn close to the Pines so that when the time was right, he'd be able to enact his revenge.

The thing about reincarnation that most people don't get though is that you aren't always reborn right after you die. Sometimes you're reborn ages after you die, sometimes you're reborn before you even ever lived. Bill didn't worry though since his power would make sure he was reborn close to the same time so he could take down the Pines family.

As it happened, Bill felt himself re-enter the world just a decade or so before he left it. He used his fading consciousness to make sure a few of his own traits would carry over to his soul's new host. He'd be smart, determined, and he'd fight tooth and nail for what belonged to him. He'd be gifted with magic, having some of Bill's leftover magic stored in him. He'd also be sensitive to the stuff. More than any normal human at least. He'd also have a little quirk. Something to set him apart from the crowd. A reborn version of himself (the most powerful being in existence) couldn't be boring and ordinary, oh no!

If Bill had had enough of his consciousness remaining when his work finished to see what quirk his reborn self had ended up with, he would have screamed.

Thirteen years - minus a summer - later, the one who was once Bill Cipher arrived in Gravity Falls with the stars on his forehead and his twin sister at his side to spend what was probably going to be a boring summer with his weird great-uncle in the rundown old house that he had transformed into a tourist trap he called "The Mystery Shack."