Castle Skull.
A witches' brew ... in a castle on the Rhine. Three inexplicable murders lead world-famous sleuth Bencolin into a strange case of twisted revenge. It was a case complicated by ghosts, old legends, werewolves, magic ... and by a fascinating list of suspects: a mad duchess, an actor with a Hamlet complex, a virtuoso who likes to play his violin in the dark, a glamorous young lady who paints in the modern manner, a dynamic Belgian financier and his beautiful, weak-willed wife, and a newspaperman whose job is to report on Europe's haunted castles. Here is a sizzling witches' brew of murder and mayhem, seasoned with the typical Carr wit, and served up in an eerie Gothic setting. goodreads review