The film was adapted by Blatty from his own novel.
The Exorcist III takes place 15 years after the original film. After a struggle, he declines Pazuzu's offer of power and Regan banishes Pazuzu, appearing in the form of locusts. In the end of the film, Regan and Father Lamont, who has been trying to help her, but has become possessed by Pazuzu, return to Georgetown. There are flashbacks of Merrin battling the demon in Regan and also flashbacks of Merrin's exorcism of Pazuzu from a boy named Kokumo in Africa many years earlier. In Exorcist II: The Heretic, Pazuzu is named as the demon and returns to haunt Regan. The majority of the film deals with Regan's demonic possession by a being she initially refers to as "Captain Howdy." The demon is ultimately exorcised out of Regan's body after Merrin dies of a heart attack, and Father Karras sacrifices himself by luring the demon into his body and then hurling himself through a window and down the infamous flight of stairs leading down to M Street NW, in Georgetown. In the beginning of the film, Father Merrin finds a ruined statue of the demon during a dig in Iraq. Two years after the novel was published, The Exorcist was released in theaters as a motion picture. In the end of the novel, the Gemini Killer leaves the body of Father Karras when Kinderman accepts that he is in fact the Gemini Killer, satisfied that his work has been recognized and his past avenged. Although not directly identified as Pazuzu, the Gemini Killer refers to "others" who would see his work continue. He does this by driving the Gemini Killer's soul into Father Damien Karras's dead body. Pazuzu returns in Legion, wanting to take revenge for being thrown out of Regan's body.
In their struggle to free Regan from the thrall of Pazuzu, both priests perish (Merrin dies of a heart attack and Karras sacrifices himself to save Regan). After Regan's mother worries about her daughter being possessed, Merrin and Karras arrive at her house and perform an exorcism on Regan and successfully force the demon out of Regan's body. The demon is later revealed to be Pazuzu though never explicitly stated to be the demon, two references were made about his statue, which was uncovered in the prologue by Father Lankester Merrin in northern Iraq.
The novel is about a 12-year-old girl, Regan MacNeil, possessed by a demon. Pazuzu first appeared in William Peter Blatty's The Exorcist in 1971.