Underworlders are the Evil races of Magical beings whose ancestral home is the Darkness that is the Underworld. While the Underworlders were originally many different races of beings that served Evil, there have been massive amounts of cross breeding over the years which have created many different Evil Hybrids. Underworlders is, in all actuallity, a term that was coined by the Blessed Generation for all beings that serve Evil and live within the Underworld.
The primary existance of many, if not all, Underworlders is to battle the forces Good and spread Chaos throughout the world through whatever mean necessary. In the Hierarchy of Evil, all Underworlders serve the The Dark One; who is Evil Incarnate. This being said, most Underworlders know no true sense of loyalty and are ultimately selfish beings that seek claim the title of the Dark One for themselves. As such Underworlders of various kinds are often involved with struggles for power & territory.