Two black magicians, brothers in the DeSade family (spelled in the IMDB as "Desard" but we see what you did there), are in conflict. Probably because they're in a film that ran out of money, was cobbled together from bits and makes little in the way of actual sense. The brothers are Lon Chaney Jr. and John Carradine and the film is known as either Blood of the Man Devil or House of the Black Death, depending on who you ask.
If nothing else--if you are pressed for time--at least watch the first part so you can hear the Lord of Darkness himself spout some poetry that makes Nipsey Russell sound like T.S. Eliot.
Thanks to Rox for finding this for us.
Oddly, the film is not available in print from what I can tell.