Insipred by the alleged true story of the Tanacu Exorcism, that happened in 2005, in Romania.
Although the demon mentioned in the film, Agares, is documented in several books throughout history, the exorcism case this film is based on believed the girl was possessed by Satan, not Agares. Additionally, Agares is best known for teaching languages and causing earthquakes but not possession.
In the real life exorcism case that this film is based on, the exorcism was actually never interrupted or halted partway. Supposedly, it was completed and she was cured but passed out afterwards when given food and water. Officially, she died of an adrenaline overdose in the ambulance on the way to the hospital. However, the details of chaining her to a cross, her family history, and mental health diagnosis remain fairly intact and unchanged in the film.