Pazuzu is a powerful demon who was based loosely on a mythological figure native to the Babylonian culture where it was considered the king of wind demons and son of the Babylonian god Hanbi. Pazuzu managed to possess Regan after she played with an Ouija board (appearing to her in form of the now-infamous demon called "Captain Howdy") turning her into a hideous vessel for it to mercilessly taunt and abuse those around itself while also displaying major supernatural powers that increased as the story progressed. It presumably grew in strength as the demon took more and more control over Reagan's hapless form which twisted her into a demonic parody of her former self which included behaviors like vigorous masturbation.