Oh dear! Despite having some big names in its cast, this very obscure and very, very early seventies movie is so muddled it doesn't even know which genre it's in. Its title and marketing imply horror, but we see precious little of the promised Satanic cult or anything else of a horrific nature, the 18 certificate being for the numerous scenes of female nudity and recreational drug use. If the idea of Oliver Reed smoking a joint scares you, this is high-octane nightmare fuel; otherwise, not so much.
Mainly it's a very clumsy social satire, presumably inspired by the fact that Longleat House could be hired as a movie location. Derek Jacobi's character seems to be a merciless parody of its real-life owner the 7th. Marquess of Bath, a mega-rich hippie with a harem of "wifelets" whose main leisure activity other than promiscuity was decorating the interior of his vast Elizabethan manor with the awful murals that feature prominently in this film (46 years on, he's still alive and hasn't changed much). As for Oliver Reed, he plays his trademark brute, dominating the entire film and everyone in it so effortlessly that his pact with the Devil seems a bit superfluous. Which is just as well, since his only magical power seems to be the ability to cause stoned or mentally unstable people to have red-tinted prophetic visions of sinister goings-on that look good on the poster but comprise about one minute of the movie.
This is the kind of film where the director can't think of anything interesting to do with the fact that his shooting location just happens to be the only house in the UK with a hundred lions in the back garden, and a fabulously wealthy aristocrat who lives in a palace serves his equally snobbish guests Bell's whisky because the prop man simply didn't give a monkey's. And although Oliver Reed's over-the-top but genuinely menacing performance is the only thing in the film worth watching, in the many scenes where he barely conceals his utter contempt for his employer under a veneer of servility about one molecule thick, he puts on that extraordinary accent used by Spike Milligan when he pretended to be a policeman and absolutely nobody else ever. It's so silly that late in the film another character actually mentions how absurd his accent is so they can pretend it was meant to be all along.
As for the subplot involving the sadistic abuse of two extremely young children, which thankfully we see very little of but hear quite a lot about, it's genuinely nasty without being the least bit interesting or even particularly relevant, except as a way to eradicate any vestiges of sympathy we might have for these horrible people. Which is hardly necessary in a film which has literally no sympathetic characters whatsoever, except possibly the children, who barely register as characters because they're so young they can't act at all and just randomly toddle about until the situation overwhelms them and they burst into tears.
I would have assumed this film to be a feeble attempt to rip off "The Wicker Man" if it hadn't been made a couple of years earlier. However, it undoubtedly does borrow a great deal from "The Servant", made in 1963 by Joseph Losey with a script by Harold Pinter and starring Dirk Bogarde. If the basic theme of a selfish upper-class man being ruthlessly outsmarted by his totally amoral employee appeals to you, "The Servant" is an infinitely superior treatment of the subject. It's not a horror film, but with a few very minor cuts, this movie wouldn't be either. It doesn't really fit into any genre, but if there was one called "slow-motion train-crash" it would be a perfect example. For devotees of deservedly obscure oddities and Oliver Reed completists only.