Marauders of Gor John Norman Chronicles of Counter Earth #9 America Star Books 1982
Tarl Cabot has struggled to free himself from the control of Gor's powerful Priest-Kings, but to no avail. Now he finds that mission challenged by a threat emanating from the planets forbidding northern lands.
There, a menacing alien force waits for Tarl, who faces an awesome choice: protect his own position as a rich merchant-slaver, or risk everything to defend the freedom of his world.