Star Barbarian, by Dave Van Arnam

Centuries before, the multiwave-drive ships had come to Morkath of the Caravan Stars -- bringing colonists, empires, confederations of plunderers. Finally, the planet was exhausted and left to descent slowly to barbarism.

Now, savage tribal leaders fight over the blighted lands. Among these warriors is the young hunter Jamnar, who vows to reclaim Morkath from the demonic powers that grip it -- the dark god Shaphath, the priestesses of Astaphar, the evil priests called Kvunuvun...

Jamnar will have help in his quest. Prosperon, the interstellar castaway, will lead him to the Forbidden Temple of Telshasoth. But only if Jamnar dares to enter the temple's portals -- and seize the 3,000-year-old secret of a lost civilization -- can he hope to save his people, and his planet, from a still greater terror...