The scenario that we're playing through (with some minor tweaks and the detail stretched out a bit more) is 'The Black Chateau' from Shadows Over Filmland by Ken Hite and Robin Laws for the RPG Trail of Cthulhu published by Pelgrane Press. It's a really solid collection of scenarios inspired by hollywood horror of the 1930's. The scenario is inspired by the 1934 film 'The Black Cat' staring both Bela Lugosi and Boris Karloff. There's a few clips from the fillm available on youtube, which I've embedded below.
The back of my (newly arrived) copy says:
The first and possibly the greatest pairing of Lugosi and Karloff is one of the darkest, most macabre horrors ever made. Dr Werdegast (Lugosi) a POW for fifteen years has been freed and now seeks news of his wife and daughter and vengeance on Hjalmar Poelzig (Karloff), the man whose betrayal leads to his imprisonment and the deaths of thousands of his countrymen during the war. He tracks him down to the castle he has built on the site of their fortress and soon discovers the diabolical secrets held within its walls. Poelzig is now the leader of a stanic cult, engaging in macabre practisees and rituals. One man's pure evil against the others tormented nead for revenge leads to an absorbing battle and a shocking climax.
The Black Chateau seemed to me one of the most immediately appealing scenarios in the collection. I liked the combination of a modernist aesthetic, with post-war desolation of the setting. Also, there's a strong player-as-author component, which was appealing and I also hoped would fit well with a play-by email format. Also, it's really good to include both an author and an actor in this setting, given it's cinematic inspiration.
