Legion (The Exorcist #2) by William Peter Blatty - Vintage Paperback

A young boy is found horribly murdered in a mock crucifixion. Is the murderer the elderly woman who witnessed the crime? A neurologist who can no longer bear the pain life inflicts on its victims? A psychiatrist with a macabre sense of humor and a guilty secret? A mysterious patient, locked in silent isolation in the psychiatric ward?

Lieutenant Kinderman follows a bewildering trail that links all these people, confronting a new enigma at every turn even as more murders surface, with each victim suffering the same dreadful mutilations. Kinderman must confront the creeping suspicion that there’s a connection between these crimes and another series of murders that took place more than a decade ago―and supposedly ended with the death of the Gemini Killer.