Las Noches del Hombre Lobo

Las Noches del Hombre-Lobo (also known as Nights of the Werewolf and Nights of the Wolfman) is a 1968 Spanish horror film about the werewolf Count Waldemar Daninsky, played by Paul Naschy. It has always been referred to as the second of Paul Naschy's 12 Waldemar Daninsky Werewolf movies. However, there are serious doubts as to whether the film was actually made; no one, including Naschy himself, has ever seen it. It was never theatrically released, nor has it ever turned up on video. It is considered today to be a lost film.
Authenticity.
Naschy was the only person who ever insisted that his script for this movie was actually filmed. He insisted in interviews that he filmed his scenes for this film in Paris. The story goes that the director, Rene Govar, was killed in a car accident in Paris a week after the film was sent off to the lab for processing, and that since no one ever paid the negative costs, the lab held onto the film for collateral and later misplaced or discarded it. To make matters even stranger, the lead actors whom Naschy attributed to this film (Peter Beaumont and Monique Brainville) apparently never existed; Naschy could not even recall the names of any other actors who starred in this film. The "French director" has no other credits on IMDb, nor is there a record of a director named Rene Govar ever working in the French film industry. This is why most Naschy fans regard this movie listing as a hoax or error, some thinking that Naschy may even have made it up to boost his resume at a time when he was just starting out in the industry. However, till the very day Naschy died, he insisted that this film once existed.
Plot.
Seeing how this is a lost film, little is known about its plot. All that is known of it, as mentioned by Naschy himself, is that the story deals with a professor who learns that a student of his suffers from lycanthropy, and under the guise of helping him, uses him as an instrument of revenge by controlling him by means of sound waves when he transforms. It is possible this film later somehow became the 1970 "La Furia del Hombre Lobo" ("The Fury of the Wolfman"), as the plots of the two films are very similar, and that would explain why "Nights" no longer exists. Someone on a fan site claimed they saw a still photo from this film, but it could have actually been a still from "Fury of the Wolf Man" with a variant title printed on it by the distributor, especially since "Fury" was only theatrically released five years after it was made.
Rest of the series.
"Nights" was followed by the third film in the series in 1969, Los Monstruos del Terror" ("Monsters of Terror").