"A kindly old grandfather is actually the leader of a murderous satanic cult which sacrifices its victims on Halloween."
How could I not do this one? A grandfather is raising his grandson (who is really his son from a night of incest) to inherit his evil powers and take over the local chapter of the guys who meet a barn a worship the devil. One of the worst/most awesomely bad movies I've seen.
Good luck finding this one. I picked it up in a 99 cent bin at a VHS clearance. There are copies out there but your going to have to dig through your local flea market if you really want to see this.