Star Barbarian (1969) a novel by Dave Van Arnam. [the back cover copy]:
"A towering new hero and an epic adventure... in a darkened world of the future: 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 descend slowly into barbarism... Now, savage tribal leaders fought over the blighted lands. Among those warriors was the young hunter Jamnar, who vowed to reclaim Morkath from the demonic powers that gripped it - the dark god Shaphath, the priestesses of Astaphar, the evil priests called the Jvunuvun... The might Jamnar would have aid in his quest. Prosperon, the interstellar castaway, would lead himto the Forbidden Temple of Telshashoth. But only Jamnar himself dared to enter the temple's portals - and therein to seize the 3000-year-old secret of a lost civilization - could he hope to save a people and a planet plunged into infamy and terror?" [This has a sequel: Lord of Blood (1970.)]