The Letter

Mason "Dipper" Pines is presented with the phenomenal opportunity to attend his dream school, West Coach Tech, but there's a catch: the only way to win a full ride scholarship is to invent something that's never before seen. Out of his own original ideas, Dipper returns to Gravity Falls to find inspiration at his Great Uncle's business, the Mystery Shack. Only, the one thing he didn't expect to find was a creature from his past: a specific, demonic triangle.
[ Billdip maybe? I think yes ]