steel worlds; (noun), an ancient race of Kurii, whose advanced technology contributed to the ruin of their planet, now share a boulder-strewn, enigmatic black space beyond the orbits of Gor, Earth and Mars, living on space ships such as the one found half buried in the Tahari. Since they have no planet of their own, they seek one, possibly Gor, or even Earth. The Priest-Kings constantly battle them, and monitor to see that they do not intrude beyond certain atmospheres, orbiting with the asteroids in their 'steel-world'.
Somewhere, beyond the fourth ring, mixed in the belt of asteroids, intruding within the perimeters refused to them by PriestKings, were the patient, orbiting steel worlds. This I had from Samos. They were nearer now. Somewhere, above that placid sky with its swift, white clouds, closer now, were Kurii
Book 11, Marauders of Gor: Chapter 28