Duergar

Duergar are Dwarves that are both emotionally & mentally unstable, and almost completely evil. Their violent streak appears when confronted with Black Elves and Dwarves. This stems back to a time millennia ago when the Black Elves enslaved their ancestors, and tortured them to breaking point. This racial memory still lingers, and has twisted the Duergar.
The Duergar still maintain strongholds, and carry out many tasks associated with their ancestor race, such as mining, smithing, and warfare. They are as tough and as skilled as any Dwarf, but in one way in these strongholds. They now look to dominate other Dwarves, who they see as not coming to their aid, or taking them in afterwards, and all other Underdark races. Each of their strongholds are filled with slaves who are worked to death, or sacrificed to any number of evil deities.
When they fled the Black Elves, they had been corrupted further by worshipping gods, and demons & devils. Each of their strongholds have a number of shrines to these, with Loki, Hel, and a god called Hashut being the most common of them. Every day Black Elves, and Dwarves are sacrificed in them, and the temples are at the core of the stronghold, with the priesthood now ruling over a king, and other noble classes below them. The temples resemble ziggurats with a constant fire burning at the top of them, where the sacrifices are thrown into whilst still alive.
They are constantly looking to expand and some believe that they have reached as far south as the northern borders of the Middle East. The problem here is that they are stretched thin in places, and they have trained some of their slaves to act as slave soldiers. After battle reports by a few survivors of battles against them is that many use Hobgoblins as slave soldiers, or mercenaries that are paid in weapons, armour and loot from battle.