Sir Christopher Lee fell over a cable and injured his back during production. On the plus side, he was also made a Knight while working on this movie.
This was Christopher Lee's first Hammer film since Une fille... pour le diable (1976) 35 years earlier. It was also his final Hammer film before his death on June 7, 2015 at the age of 93.
Shot over a period of thirty days.
The main plot is almost identical to a Spanish movie released on the same year: Malveillance (2011).
Director of photography Guillermo Navarro worked for half his usual fee because he was intrigued by the possibilities presented by this movie.