The Devil Rides Out (1968), probabily the best film Hammer ever produced, this tale revolves around a Devil worshipping cult led by Mocata (Charles Gray), and they have Simon (Patrick Mower, later Rodney Blackstock in Emmerdale :P), who is a young friend of Duke of Richeleau (Christopher Lee), who has an acquired knowledge of the dangers of the dark arts, and he might be as powerful as the Devil himself. A very creepy and surprising film, although some of the special effects have dated badly, it's still a great pleasure to watch, and does emit some shocks. 5/5
