The Cometeers — Jack Williamson

In the second book in the Legion of Space series, Jay Kalam, Hal Samdu, and Giles Habibula fight The Cometeers, an alien race of energy beings controlling a "comet" which is really a giant force field containing a swarm of planets populated by their slaves. The slave races are of flesh and blood, but none are remotely similar to humans. The Cometeers cannot be destroyed by AKKA, as they are incorporeal from the Universe's point of view and exist for the most part in an alternate reality.

The ruling Cometeers feed on their slaves and literally absorb their souls, leaving disgusting, dying hulks in their wake. It is said that they do so, as they were once fleshly entities themselves of various species. Hence, the ruling Cometeers keep other intelligent beings as slaves and "cattle." They fear AKKA, though, as it can erase all their possessions.