Disclaimer: I don't own Harry Potter or Gravity Falls.

/wat

Bill Cipher materialized in the house, letting out an amused cackle as he heard the screams of someone dying. It was like music! How fortunate that in the first randomly-selected home, people were screaming? A baby's wails filled his purely-figurative ears, and his singular eye watched as the green light being emitted from the doorway died down.

One last strangled cry from an adult, and the continued screeching of a wee babe.

His curiosity was piqued. The man's screams were squandered, but the baby lived on? Bill got a strange feeling, and it turned to glee as he floated into what was undoubtedly a nursery. "Dark magic!" he exclaimed happily, recognizing the power still echoing off the walls. "Oh, you wizards always know how to make my day." Bill leaned over to examine the pulpy mess that was, until recently, a wizard. He emitted a low whistling sound. "Smells like this guy was dealing in souls."

Bill had a little experience with souls, himself.

Chuckling fondly, Bill took a moment to inspect the baby. Its – well, 'his', if you want to be pedantic about it – forehead was bleeding profusely. Tutting, Bill summoned a cow-tongue to lick the blood away. With the mess gone, Bill could see the thin, lightning bolt-shaped scar. With his powers, he could see the insane darkness settling inside the child. "Wonderful!" crooned Bill. He grabbed the baby by his clothes and tucked it under his arm. "You'll make for one dandy lackey, once I fatten you up."

Bill took a moment to entertain a certain mental image: those Pines and that McGucket nerd, trying to hold him down and restrain him, when suddenly – BAM! A baby filled to the brim with a psychopath's soul is chucked at them. The daydream evolved to show the infant blasting them with magic, and Bill giggled.

"You and me, kid, we're going places," commented Bill. Eager to return to Gravity Falls and teach his new assistant every thing he could about the Darkest – not just 'Dark' – Arts, Bill opened a wormhole and floated through. By this time, the baby had stopped crying and seemed content to go wherever the strange, triangular stranger took him.

Not two minutes later, a greasy-haired man burst into the room and looked around wildly, eyes resting on the corpse of Lily Potter. A strangled sob escaped his throat, and Severus Snape went to her. It wasn't until a fair five minutes afterwards that he realized there was no Harry Potter to be found.