Nights of the Werewolf (Or his original Spanish title, Las noches del Hombre Lobo) was a 1968 Spanish/French horror film directed by René Govar and written and starring by Jacinto Molina Alvarez, known as Paul Naschy. 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.
Unfortunately, very little information on the film has surfaced. Even standard details such as cast and crew are controversial because there isn't information about them except the same Paul Naschy.
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.
In the documentary dedicated to the legacy and memory of Paul Naschy, "The Man Who Saw Frankenstein Cry", it is shown a supposed picture of that lost film.