The Dark Eldar are a subspecies of Eldar who also function as a separate faction and society. The Dark Eldar originate from the Eldar who, unlike those aboard the craftworlds who adhere to strict lifestyles to avoid a repeat of their civilisation's downfall, continue to indulge in sadistic hedonism, regularly abducting and torturing members of other races for the sake of pleasure, and to prevent the Chaos god Slaanesh from consuming their souls, the fate of all Eldar who remained on their homeworld during the Chaos god's birth.
After achieving an interstellar empire boasting fully automated industry, the Eldar species had infinite opportunity to engage in hedonistic pursuits. As the majority of the species gradually became more depraved in their constant pleasure seeking, their increasingly degenerate and excessive activities, combined with the Eldar's psychic connection to the Warp, resulted in the creation of the chaos god Slaanesh. Immediately upon its creation, Slaanesh instantaneously purged almost the entire Eldar species from the galaxy, the only survivors being those aboard the Craftworlds or left the empire beforehand out of disgust and became Exodites.
However, another faction of Eldar survived the fall besides the Exodites, Harlequins and Craftworld Eldar. Within the Webway, the Eldar city of Commoragh remained largely untouched by Slaanesh. Unlike the remainder of the Eldar race, these Dark Eldar refused to change their hedonistic ways, maintaining the social chaos that resulted in the species' downfall to begin with.
After the fall, the Eldar became burdened with the curse of Slaanesh claiming and devouring their souls upon their death. However, the Dark Eldar have been more physically affected by this. Over time, Dark Eldar will have their souls gradually drained from them by Slaanesh. In order to counter this, the Dark Eldar will seek out and abduct as many sentient lifeforms from across the galaxy as possible and use them to restore themselves via extreme acts of torture and experimentation. This, along with the Dark Eldar's clear enjoyment of such action, has resulted in them being viewed by the galaxy at large as a race of extremely sadistic pirates and psychopaths.