According to scholar Jeremy Black, demons in Mesopotamia evolved over the years from representations of threatening animals to personifications of the threat of danger and death.
Black claims that Pazuzu is the ultimate expression of this evolution and provides a simplified chronology of the progress, breaking it into five phases:
1. A formative phase, in the late Ubaid and Uruk Periods, when the features of different animals were first combined into unnatural composite beings.
2. An optimistic phase, in the Akkadian Period, when glyphic scenes show the capture and punishment of nefarious demons.
3. A balanced phase, in the Old Babylonian Period, when cylinder seal designs often mix images (gods, symbols, and other motifs) of good and bad associations with respect to mankind.
4. A transformative phase, with Mitannian, Kassite, and Middle Assyrian Art of the fourteenth to eleventh centuries BCE, when the human-centered imagery of the Old Babylonian Period gave way to a preponderance of animal-headed hybrids.
5. A demonic phase, represented by Neo-Assyrian and Neo-Babylonian art, when individual demons were depicted in their full horror.
This last phase of development accords well with the new theology of a demonically populated underworld in the first millenium BCE. The change happens, moreover, at the same time as the advent of the practice of erecting in palaces and temples monumental statues and reliefs of magically protective beings, and of burying small clay images of them in the foundations. This evolution continued into the Hellenistic Period of Mesopotamian history and carried on into the Christian period.
The Christians no longer had a need for protective demons and, of course, reliance on earlier religious beliefs was discouraged by the new faith. Demons, along with the old gods, had no place in the heaven of the Christian god and so were relegated to the Christian hell. Demons were already associated with the underworld and just as it was an easy step to turn the pagan afterlife into a hell of punishment, so it was to make demons agents of that eternal punishment as well as difficulties and dangers during one’s life.
The gospels of the New Testament depict Jesus Christ routinely driving demons from various individuals and the Book of Acts, Revelation, and others describe demons in the service of God’s adversary, Satan…Demons were thereafter considered only as agents of evil, incapable of good save only inadvertently in serving God’s overall plan. Pazuzu, an ancient Mesopotamian figure of fearsome aspect, was the perfect choice as the antagonist in The Exorcist as the audience had been primed through almost 2,000 years of instruction to accept the ancient demon-god as an instrument of evil. To the people of his day, however, Pazuzu was regarded as security and a shield against misfortunes in an uncertain and often frightening world.