With the original title, "Der Würger von Schloß Blackmoor", this movie was released in 1963 and was filmed in Germany and directed by Harald Reinl. IMDB has the following description of the plot:
"A strangler is loose on a British estate, and he not only strangles his victims but brands an "M" onto their foreheads before he decapitates them."
Has anyone seen this movie? I doubt that Dave Arneson drew any inspiration from this movie, but you never know! Also, the DA modules includes the NPC Skandros the Strangler. Could he have been based on the character from the movie?