Sherlock Holmes and the Shadwell Shadows (The Cthulhu Casebooks)

In London's East End, an outbreak of insanity sees ordinary men and women reduced to gibbering wrecks, and a mysterious creeping fog hides terrifying apparitions within it. Sherlock Holmes deduces a connection between these sinister "shadows" and an Oriental drug lord. Yet there are even more sinister forces at work, as the great detective faces a challenge so fearsome and deadly that his career may be over almost as soon as it has begun.