Complex sentence: Slavery was widespread in the Hyborian world and Howard points it had the unintended side-effect of slaves mating with their masters and producing bloodlines of mixed heritage in most Kingdoms of the time, Hyborian or not.
Split into simpler sentences: Slavery was widespread in the Hyborian world, creating the unintended side-effect of slaves mating with their masters. This produced bloodlines of mixed heritage in most kingdoms of the time, Hyborian or not.