According to director Piers Haggard the scene of the coven's attack and rape of Cathy Vespers was completely unplanned and created mainly during the shooting. Haggard said even the coven's chant was written right on the spot.
The film was originally conceived as three stories that would play out separately, but all have the unearthed remains of Satan being the linking factor between them. The stories of Peter Edmonton and his mad fiance, the possessed village children, and the Judge's battle with evil were all at first supposed to take place independently. However, when the script was rewritten, it was decided that the plots should be combined to create one central story.
At one point, both Peter Cushing and Christopher Lee were suggested for the role of the Judge but were considered to be too expensive. Other actors discussed included Donald Pleasence and Michael Gough.
For its US release, the film's nudity was censored, particularly in Linda Hayden's seduction scene by darkening the footage to avoid an X rating.
The scene where 'Satan's skin' is cut off of Margaret was inspired by an actual event from screenwriter Robert Wynne-Simmons' childhood when a doctor performed an operation on him while he was laid out on a kitchen table.