Shadow of a Dark God

From the introduction:

It is the end of March in 1892, the moon is full, and as seems to happen so often, a series of brutal murders has shocked London. The press has already begun to play up the four deaths in the heart of the city. They are particularly interesting because they are characterized by the horrible mutilation of the bodies. Scotland Yard has already initiated an investigation under the direction of Inspector Brian Bollard, and the people are afraid to walk the streets by night. Few details of the cases have been released to the press, but weird horrors are hinted at in pubs and smoking rooms.