In the 1973 film The Exorcist, the demon possessing Regan MacNeil is Pazuzu.
Pazuzu is a demon from ancient Mesopotamian mythology, and The Exorcist uses that name for the entity associated with Regan’s possession. The film does not initially identify the demon through a simple on-screen introduction; its identity is connected through archaeological imagery, dialogue, and the wider mythology adapted from William Peter Blatty’s novel.
Pazuzu is often confused with demons from other horror franchises. Valak is the demonic figure associated with The Conjuring 2 and The Nun, while Pinhead is the lead cenobite of Hellraiser. The 1973 film stars Linda Blair as Regan, Ellen Burstyn as Chris MacNeil, and Max von Sydow as Father Merrin. Its depiction of possession became one of the most influential images in supernatural horror.