Sherlock Holmes vs Cthulhu, the ultimate showdown! This is the selling point of A Study In Emerald, the game by Martin Wallace, if not quite the plot of the Neil Gaiman short story.
A little background is necessary. In 2003 there was a short story collection called Shadows Over Baker Street. In it, 18 different authors mash together Arthur Conan Doyle’s Sherlock Holmes and H.P. Lovecraft’s Cthulhu mythos in a nightmarish alternative history.
Neil Gaiman wrote the first story in the collection, beautifully introducing the concept and twisting the reader about like a confused puppet. His story, which won a Hugo Award, is available as a free PDF from his website and is worth a read if you’re a Sherlock fan – the style and in-jokes are clearly targeted at fans of Doyle rather than Lovecraft.