What demon from Assyrian and Babylonian mythology is battled by Father Damien Karras in "The Exorcist" film series? 
Lankester Merrin:  Merrin does not appear again until much later in the novel, when he joins the protagonist, Father Damien Karras, in Washington, D.C. to exorcise the demon from the body of a young girl. Pazuzu (The Exorcist):  Blatty derived the character from Assyrian and Babylonian mythology, where Pazuzu was considered the king of the demons of the wind, and the son of the god Hanbi.
Pazuzu