Satanism, torture, and necrophilia - oh my! And all in a movie made in 1941. Lugosi plays Dr. Vitus Verdegast, who is on his way to the place where he once served as a soldier. He shares a compartment with a honeymooning couple, Peter and Joan. While taking a bus to their hotel, the bus crashes during a winter storm, leaving Joan injured. Verdegast takes Peter and Joan to the nearby home of a former acquaintance (and, it turns out, mortal enemy), architect Hjalmar Poelzig.
This movie pushes all the classic "stranded in a strange house" and "dependent on a mysterious and creepy stranger" buttons in the plot with Peter and Joan. Meanwhile, Verdegast makes some horrific discoveries of his own, in terms of what happened to his wife and daughter. There is a tremendously creepy scene depicting a Satanic black mass, and there is a very subtle reference to necrophilia. The most famous scene in the film is when Vergegast finally gets the upper hand over Poelzig, chains him up, and (in shadow, of course - this was still the 1940s) starts to cut Poelzig's skin off.
The interaction between Lugosi and Karloff is really spectacular, and the film pushes the envelope of "sick and disturbing" pretty far for a movie of its time. It's still fairly tame, though, and like Rebecca, this is one you might be able to actually stomach if regular horror fare is not your cup of tea.