Pretender — Piers Anthony; Frances Hall

The ship was doomed. The host's body was dying.

The only hope was to crashland on a nearby hosted a galactic observation station; a station primitive planet which, unknown to its natives, where he could find shelter and, ultimately, rescue. But with his host's last thread of consciousness, NK-2 saw that the city he expected lay in ruins. How could he find his fellow galactics in such chaos? How could an ancient being of pure energy survive, alone on such a world?

Then salvation appeared in the form of a young boy, and NK-2 took his chance.

Now slave and alien inhabit the same body, and the boy has become a spear, thrust at the very heart of the great Babylonian Empire. For NK-2's plan requires that he become a God.