The Exorcist III (1990)

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