Hammer was originally due to film "I Am Legend" (the Richard Matheson novel) under the title "Night Creatures", but this was abandoned when the BBFC informed them that they would not pass the film. As Hammer had promised the U.S distributors a film called "Night Creatures" the title was passed on to the already completed Night Creatures instead.
The film was based on Russell Thorndike's "Doctor Syn - A Tale of Romney Marsh," but Dr. Syn's name was changed to Blyss to avoid legal problems with Walt Disney, who claimed exclusive rights to Thorndike's stories.