Title: Pretender
Description: From back cover Tor paperback June 1985:

The ship was doomed.  The host's body was dying.  The only hope was to crashland on a nearby primitive planet which, unknown to its natives, hosted a galactic observation station; a station 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.
Subjects: Fiction, science fiction, general, Fiction, fantasy, general