The Dark Eldar were once Eldar at the peak of their civilization. However, the Eldar became corrupt due to their emotional lust, which resulted in the birth of Slannesh, one of the Chaos gods. The Dark Eldar revel in piracy, enslavement and torture, and are sadistic in the extreme. The Dark Eldar are unique amongst the Warhammer 40,000 factions in the sense that they do not occupy many planets, but rather one dark city - Commorragh.
4th Generation of the Commorragh
During the great war, the Dark Eldar split from the Eldar entirely, and wiped out civilizations one by one. They were prosperous in the sense of violence and rage, as they considered their strength in war to be their strong points as a race.