Dark Eldar
Dark Eldar are a Chaos-corrupted offshoot of the Eldar, descended from the Pleasure Cults that brought on the fall of Eldar civilization.
Attributes
Dark Eldar are physically like their uncorrupted counterparts, but chaos corruption has had massive consequences. Psychologically, Dark Eldar are notable for extreme sadism and aggression. Dark Eldar are also notable for their need to consume the souls of other life-forms in order to avoid being consumed themselves by Slaanesh. These two attributes drive the Dark Eldar to launch constant raids against inhabited worlds for slaves which they use as playthings for their sadistic abuse and consumption.
The Dark Eldar are based in Commorragh, a massive city based in the Webway, although they do have minor presences outside in the physical universe. This limits the total of the effect of Slaanesh upon them and makes them next to impossible to attack. Never the less, their presence is rather minimal due to their small numbers.