The Demon Who Stole My brother
After a small deal with Bill, Dipper is taken away for 6 years of his life. After some time the rest of the gang all accepts to accept the fact that Dipper is gone. But when news arrives the Dipper is back, they take the long journey back to Gravity Falls only to see that the Dipper they wanted was the one they didn't get. And in order to keep the world safe, they must do things they never thought they would have to, or, try to bring him back to reality and sanity.