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On July 18, 1981, Ed and Lorraine Warren were present when an exorcism was performed on little David Glatzel. The priest and the boy's family tried unsuccessfully to exorcise the evil entity, Ed had a heart attack, and then Arnie - David's older sister's boyfriend - offered his soul to the demon.
A short while later, Arnie kills his landlord and afterward says he has no memory of what happened. For the first time in U.S. history, a murder suspect claims to have been possessed by a demon, and now the guy's fate depends on whether the Warrens can find proof of it.