A woman who experiences unsettling feelings (Candace Glendenning) travels to her uncle’s enigmatic estate in the countryside (Michael Gough), where a distressing event occurs and she remains to recuperate, developing a connection with her cousin (Martin Potter). However, strange and sinister events are unfolding. Barbara Kellerman appears as the secretary at the property. “Satan’s Slave” (1976) is a British film that shares plot elements with "The Witches," also known as “The Devil’s Own” (1966), and "Curse of the Crimson Altar,” or “The Crimson Cult” (1968). These films all start and conclude with scenes involving occult practices, while the central portion consists primarily of slow-paced events set in a rural environment. It also draws comparisons to "The City of the Dead" (1960) and "The Wicker Man" (1973). Similar to the latter film, there are instances where a woman is depicted completely nude—a word of caution. The satanic ritual sequences echo those in "The Devil Rides Out" (1968). Although this one is the least well-known among these films, it’s roughly equivalent in quality to the others; however, individual reactions may differ. It's a film where people who seem foolish perform rituals in old houses or dark forests while wearing hooded robes and offering naked women on unholy altars. I would prefer "The Crimson Cult," “The City of the Dead,” and even “The Witches” to this one, but it’s still superior to “The Devil’s Rain” (1975) and "The Brotherhood of Satan" (1971). Candace Glendenning, who was 22 years old during filming, portrays a character around the age of 20. She evokes memories of Kathryn Hays, only considerably younger. Potter possesses an intense look reminiscent of a rock star. The film’s length is 1 hour and 26 minutes, and it was filmed in Pirbright, Surrey, England, located just southwest of London. GRADE: B-

Source on TMDB