Forgotten Classic: The Exorcist III
https://culturefixuk.blogspot.com/2011/07/forgotten-classic-exorcist-iii.html

The Exorcist III ignores the events of the second film and reunites us with Lieutenant Kinderman (Now in the form of George C. Scott) and Father Damien Karass, both from the first Exorcist film. The film takes an interesting twist, putting us the perspective of the police trying to hunt down 'The Gemini Killer', a murderer who's attacks have a satanic undertone to them.
Blatty's direction is incredibly dark and stylish, giving the film a sinister, chilling vibe. I think partly the reason this is so overlooked is because it's a very intelligent film, marketed to the wrong audience. Unfortunately the studio demanded constant recuts and the addition of more 'mainstream horror' to make it more audience friendly. This means throughout the film there are somewhat out of place SFX sequences, which feel forced just to connect the film to the Exorcist name.
However, Exorcist III or as Blatty prefers, Legion, is an incredibly dark and atmospheric crime thriller with some chilling psychological horror - most certainly an honorable follow-up to The Exorcist.