The film is based on a real-life criminal trial.
'The Conjuring' films are all supposedly 'based on true events' drawn from the case files of controversial demonologists Ed and Lorraine Warren. The most recent film in the series, 'The Conjuring: The Devil Made Me Do It' is about a case the Warrens were consulted on where a defendant used possession as a defense during a murder trial.
In November 24, 1981, Arne Cheyenne Johnson was convicted of first degree manslaughter for killing his friend and landlord Alan Bono (named Bruno in the film.) He claimed that a demon previously exorcised from an 11 year old boy by the Warrens had taken up residence in his body and compelled him to murder his landlord by stabbing him. The defense didn't work as the judge ruled that it couldn't be proven and Johnson was sentenced to 10-20 for first degree manslaughter, of which he served 5 years.
The parents of 11-year old David Glatzel reached out to the Warrens after their child began acting erratically. After attempting an exorcism, the Warrens said that Arne was present at the exorcism and afterward he began to exhibit the same behaviors that Glatzel did. Meanwhile, Johnson's brother said that his brother was mentally ill when he committed the crime.
The major change between the real life case and the story is that the defense didn't actually use a "the devil made me do it" defense." Instead, they based their defense on a self-defense plea. Also the details behind the Occultist, the evil force behind everything, is also fiction.
According to a Fangoria interview with the film's writer David Leslie Johnson-McGoldrick, the investigations in the second half of the film were drawn from the Warrens' experiences as investigators. "We could start off with Arne's story, and then we moved into composite territory where we were plucking stories of her investigations and folding them into this story," he explained. "And that gave us a little bit of license in terms of coming up with things that were not necessarily from Arne's story, but were still based on their case files."
The film is currently streaming on HBO Max.