Dominion: Prequel to the Exorcist- although Blatty was not involved, he approved of the film and told The Houston Chronicle that the film is "a handsome, classy, elegant piece of work"
The Exorcist (TV Series)- The Exorcist television series is considered to be semi-canon because it is based off the novel and differs slightly from the films. It also follows a separate timeline than the films and/or novels. Blatty was not involved, however, he visited the set in 2016 and according to the series creator, Jeremy Slater, "We have the rights to [William] Peter Blatty's novel and he's not very involved creatively but, he's been pitched and briefed on everything. He's incredibly excited. We can't wait for him to come visit on set because this really is his baby. This is the world he created and right now we're just trying to do it justice."
Exorcist II: The Heretic - William Peter Blatty had no involvement whatsoever. Warner Bros. was in a rush to compete with 20th Century Fox's The Omen, released in 1976. Blatty hated the film and it was Warner Bros. doing their own thing without his involvement.Legion/The Exorcist IIIignores the events of Exorcist II and is considered to be the "true" sequel.