Legion by William Peter Blatty
Legion is a 1983 horror novel by William Peter Blatty, a sequel to The Exorcist. It was made into the movie The Exorcist III in 1990.
Like The Exorcist, it involves demonic possession. The name is derived from The Bible, particularly The Gospel of Luke, which describes Jesus traveling in the land of Gadarenes where he encounters a man possessed by Demons:
“ Jesus asked him, saying, "What is your name?" And he said, "Legion," because many devils had entered him. (Luke 8:30) ”
Or the more common quote on the incident, sometimes called the Gerasene Demoniac, from The Gospel of Mark:
“ And he asked him, "What is thy name?" And he answered, saying, "My name is Legion: for we are many." (Mark 5:9)
The story opens with the discovery of a twelve-year-old boy who has been murdered and crucified on a pair of rowing oars. Kinderman already sees that the boy is mutilated in a way identical to the victims of a serial killer known as the Gemini Killer, who was apparently shot to death by police twelve-years previously while climbing the Golden Gate Bridge in San Francisco. A priest is later murdered in a confessional, once again bearing the mutilations distinctive of the apparently deceased killer. The fingerprints at the two crime scenes differ, however. Further victims soon follow, including one of Kinderman's friends, another priest, who is slain in a hospital, his body drained of blood before being decapitated. Yet again the Gemini Killer's mutilations are present.
In the end, the implication is that the Gemini Killer possessed the body of Damien Karras and spent many years trying to gain control of the body, during which time Karras was held in a mental hospital. He lacked any identification and was nicknamed Sunlight because he sat in the sun's rays as it passed through the window of his cell. Upon finally gaining control of Karras' body, the Gemini occasionally left it to possess the bodies of the patients suffering from senile dementia, and as they were in an open ward with access to the outside world, he could use them to go forth and commit murders. This is why the fingerprints of several senility patients were found at the crime scenes; their bodies carried out the murders but the Gemini Killer was in control of them.