Duergar are a race of dwarves that mostly populate the Underdark, the interconnected caverns deep underneath the earth. It is said that the original Duergar broke from the dwarves of the surface due to the god they worship, who broke from the dwarven pantheon a long time ago; nowadays though it's not uncommon to find duergar who also broke from their own pantheon, with at least 50% of them having returned to worshiping Moradin. Bad run-ins in the Underdark with Mindflayers deeply changed the duergar, though, giving them minor psychic powers and resistances.