Complex sentence: In addition, the prequel novels of the "Word/Void" and "Great Wars" series contain demons (uncapitalized in the texts) of an apparently different origin--humans that have sold their souls in service to the Void.
Split into simpler sentences: In addition, the prequel novels of the "Word/Void" and "Great Wars" series contain demons (uncapitalized in the texts) of an apparently different origin. They are humans that have sold their souls in service to the Void.