Despite the claims made throughout the marketing and in the film itself, this is not based on "actual accounts". While there really is a Ralph Sarchie who really was an NYPD officer who claims to have encountered paranormal phenomena in his work and who co-wrote an allegedly non-fiction book that "Deliver Us from Evil" purports to adapt, the film does not actually adapt any of the cases found in said book and instead features a wholly original plot imagined by director Scott Derrickson and his co-writer Paul Harris Boardman.
Early in the film, Sarchie's arm is slashed by Jimmy in an encounter, suffering a deep gash that it is stated will require stitches. In the very next scene at the Bronx Zoo, the wound is not visible. Immediately following, we see him at home and the arm is bandaged.