The great Richard Matheson does solid work turning a novel by the often clunky Dennis Wheatley into a crisply executed occult thriller that clocks in at barely 100 minutes. Lee commands the screen as a reluctant, learning-on-the-fly white magician who must battle the powerful Mocata (a terrifically oily, ingratiating Charles Gray) for the souls of two people who have been pulled into Satanic worship.
The rites and spells sometimes sound so odd that you'd swear they were lifted from H.P. Lovecraft or William Hope Hodgson's Carnacki the Ghost-Finder series and not from actual occult sources. This Hammer film has a fairly low budget, as Hammer films always did, but the cinematography, direction, and set design mostly make up for it. There are a couple of goofy moments involving visual effects, but a couple of things also work quite well.
The film is at its creepiest when it keeps its demons off-stage, but that's true of virtually all horror movies. Wait for the moment in which a crucifix operates pretty much like the Holy Hand-grenade of Antioch. Reportedly this was Christopher Lee's favourite of his many Hammer Horror Films, partially because he himself suggested they make it and partially, I assume, because he got to be a commanding good guy for once. Recommended.