S1·E23 True Believer

A non-believer in the supernatural, Detective Frank Sheen must investigate a priest's suspicious suicide. Upon questioning Father Dowling he learns that the convent is demonically possessed. Unbeknownst to the detective, the site used to be a convent that closed down when a Sister Theresa committed suicide. After one night in the place, he soon begins having his own suicidal feelings.