Screenwriter Christopher Wicking thought that the original novel by Dennis Wheatley was extremely bad and had no compunction about changing it extensively. His script was, in turn, rewritten by others, mostly during shooting, and he pronounced himself dissatisfied with the film, especially the ending.
The trivia item below may give away important plot points.
The film's abrupt conclusion was not the original ending. In the original script the character of Father Rayner recovers from the rock, thrown by John Verney, and attempts to pursue Verney and Catherine. He is then struck by a flash of lightning as he crosses the circle. Although this ending was filmed none of the footage survives and is illustrated on the DVD by monochrome stills.