This film has been out of circulation in the U.S. for several years (even to the extent that it has never been officially released on home video in any form in the U.S.) after Warner Bros. sued American International Pictures, claiming that it was a ripoff of their film, The Exorcist (1973). Although the film was very profitable for AIP (its budget was approx. $200,000 and it earned $4 million in its first month of release), its producer/director William Girdler never saw one cent of the profits due to the lawsuit, which was finally settled shortly before his death in 1978; however, it turned out to be too little, too late as far as the film was concerned.
Carol Speed got the part of Abby Williams in the film after the original actress demanded a masseuse for which its low budget had no provisions. Speed's agent recommended her to the film's producer/director William Girdler and she flew to Louisville, KY, meeting Girdler for the first time on its set.
In addition to the devastating tornadoes that struck during filming, there were instances of bad luck on the film's set. Terry Carter broke his ribs while filming one scene and whenever Carol Speed would appear on the set dressed as the demon, the generators would fail.
The song that Abby Williams sings in church, "My Soul Is A Witness", was written and performed by the actress herself, Carol Speed.
Filmed in Louisville, KY, the film's production was temporarily halted when a series of tornadoes ripped through Louisville on April 4, 1974. Carol Speed recalled taking refuge with Juanita Moore in the lobby of a hotel that had no tornado shelter. They weathered the storm wrapped in blankets to protect them in case one of the large glass windows in the lobby was shattered.