The Exorcist III (movie, 1990)

Set fifteen years after the original film, The Exorcist III centers around the philosophical Lieutenant William F. Kinderman who is investigating a baffling series of murders around Georgetown that all contain the hallmarks of The Gemini, a deceased serial killer. It eventually leads him to a catatonic patient in a psychiatric hospital who has recently started to speak, claiming he is the The Gemini and detailing the murders, but bears a striking resemblance to Father Damien Karras.