Duergar

The Duergar are those kindreds of the dwarves who have forsaken the worship of Urugan, ancestral father-god and creator of their race, and instead devoted themselves to the gods of the underworld. They are a cruel, proud, and power-hungry breed; in contrast to the loathsome, creeping Derro, many are formidable warriors and some are masters of dark sorcery thanks to the favor of their infernal deities. Fortunately they, like the Derro, are few in number and scarcely seen. 
How the first Duergar came to be is unknown, but legends say that they were dwarves who--mining ever deeper towards the earth's molten bowels in search of rare ores and gems--discovered gateways to Karkharos in the uttermost depths or in the hearts of volcanic pits. Through these portals they met or summoned the denizens of the netherworld, and though some surely fled in terror or were slain or dragged down to eternal torment, a few of their number made pacts with the dark gods and willingly entered their service instead. 
The name of the Duergar is the shortened form of a longer dwarven phrase, which means "Breakers of the Fundamental Oath" or "Violators of the First Covenant."