Legion: William Peter Blatty

Book III of The Exorcist. A young boy is found dead by the side of a river, horribly crucified. Detective Bill Kinderman is baffled by the brutality but is more shaken by the mutilation that links this case to the 'Gemini' killings of the late 60s. Three days later, Kinderman is recording another death at Georgetown General Hospital - this one as bizarre as any he's ever encountered. He begins to piece together a solution to the new killings and the unsolved earlier deaths. The 'Gemini' killer is believed to be dead, shot down in a hail of bullets on the Golden Gate Bridge - or is he?