Out of circulation for years after Warner Bros. sued American International claiming it was a ripoff of its film The Exorcist (1973). Although the film was very profitable (its budget was approx. $200,000 and it earned $4 million in its first month of release), director/producer William Girdler never saw a penny of it because of the lawsuit, which was settled shortly before he died in 1978.
Filmed in Louisville, Kentucky, the production was temporarily halted when a series of tornadoes ripped through Louisville on 4 April 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 exploded.
In addition to the devastating tornadoes that struck during filming, there were more instances of bad luck on the set. Actor Terry Carter broke his ribs filming one scene, and whenever Carol Speed would appear on the set dressed as the demon, the generators would fail.
Carol Speed landed the part of Abby after the original actress demanded a masseuse, for which the film's low budget had no provisions. Speed's agent recommended her to director William Girdler, and she flew to Louisville, meeting her director for the first time on the set.