The Demon Among Demigods

When Percy and Annabeth are sent to a small town in Oregon after recieving an ominous prophecy, they aren't sure what to think. Gravity Falls is strange, that's for sure, but something is off about the creatures that live there. They aren't so much as targeting the demigods as targeting everyone. Soon, they meet the family Pines, and when the demigods and pines twins discover what they're up against, they realize that the only way to win is to work together.