The Dark Eldar are unique amongst the races in the sense that they do not occupy many planets, but rather one dark city called Commorragh. They are mainly pirates, though are sometimes used as mercenaries.
The origins of the Dark Eldar can be found in the Fall, the great cataclysm that nearly destroyed the entire Eldar race. It was an event so terrible that not only did it kill trillions of Eldar, but it breached the gap between real space and the warp, and gave birth to a Chaos God.
To understand the reasons for the Fall, it is necessary to know something of the Eldar mind and soul.
An Eldar's mind is incredibly complex. Their senses are extremely sharp, able to perceive incredible levels of detail. Their emotions can be so strong that a human’s are merely pale shadows by comparison. They are extremely intelligent; their thought processes are much faster than a human’s. All of this means that an Eldar experiences the universe and all its sensations to a greatly heightened degree.
Similarly, an Eldar's soul is much brighter in the Warp than those of 'lesser' sentients. Eldar are able to affect the nether-realm much more than most other races. They are all latent psychics and have the ability to become very powerful psykers with training. It is the strength of their souls that was one of the causes of their downfall.
Before the Fall, the Eldar had an immense galaxy-spanning empire comprising millions of worlds, larger and more powerful than even the Imperium of Man at the height of its power. The Eldar lived in relative peace—barbarian races such as the Orks were kept at easily manageable numbers and never had the strength to threaten the might of the Eldar empire. The humans were not yet virulently xenophobic and did not have a large domain, and the Tyranid Hive Fleets were unknown. The C'tan and Necrons, ancient foes of the Eldar, were long ago defeated and still remained dormant.
Life on the Eldar worlds was idyllic, with fantastically sophisticated machines to take care of all labour and manufacturing required, leaving the Eldar free to indulge in other, more aesthetic pursuits. With all menial work taken care of for them, the Eldar became indolent and decadent. They began to explore more and more the arts of pleasure, delving ever deeper into hedonism. The descent into decadence spanned millennia. Tradition and order disintegrated as they limited the pursuit of pleasure. Sects called Pleasure Cults were formed, dedicated to achieving the highest levels of hedonistic sensation, and their ceremonies and practices became ever more wild, eventually devolving into violence and sacrifice of their own kind. Some Eldar hated what their race had become and left the Homeworlds for the virgin Maiden Worlds, or left on the newly-constructed Craftworlds, leaving the Pleasure Cults to their madness.
Meanwhile, something terrible was stirring in the Warp. The millennia of Eldar hedonism had made a massive impact in the psychic realm of Chaos. Within the warp the decadent Eldar civilization was giving shape to a Power of Chaos, which grew and grew over thousands of years, getting stronger and more defined until suddenly it sparked into an intelligence – a shatteringly huge and malign intelligence, with an immense and bottomless thirst for Eldar souls. This was the birth of Slaanesh.
The process lasted for thousands of years, corresponding to mankind's Age of Strife, although when Slaanesh finally came into being, the results with the universe were apocalyptic and sudden. An almighty psychic shockwave scythed across the galaxy. The souls of almost every Eldar were stripped from them in an instant and devoured by the new-born Chaos god. There were few survivors. Most were driven mad, their minds trapped half in the real world and half in the swirling insanity of the Warp. A great warp rift was created, encompassing the entire Eldar empire and creating the Eye of Terror.
Of the few survivors of the Pleasure Cults, some remained sane and able. They found to their horror that Slaanesh had not yet finished with the Eldar – S/he was slowly draining their souls while they still lived. To escape this these survivors fled in small groups to the Webway, the system of sealed tunnels that run through the Warp and enabled the Eldar to traverse the galaxy safe from attack by Daemons and other Warp creatures. Deep in the Webway, these small groups came together and laid the foundations of Commorragh. More and more survivors began to arrive, and added their own parts to the new city, making it even larger and more heavily populated.
Some Kabals can trace their history back to the creation of the Dark City, including Asdrubael Vect’s Kabal of the Black Heart.
Dark Eldar, like most Eldar, make use of advanced technology, including anti-gravity devices, dark matter weaponry, nanotechnology and psychic artifacts. While Dark Eldar do make use of psychic devices, they do not use psychic powers themselves. Psykers are treated as playthings in Commorragh, and given the twisted sadistic nature of the Dark Eldar, this necessarily involves pain and torment for the psyker. The technology, weapons and wargear of the Dark Eldar are analysed in the Dark Eldar Armory.
Over time, Dark Eldar begin to suffer more and more from The Thirst. They develop an all-consuming and ever-increasing need to drink the souls of other beings. It is postulated that the cause of this is the Chaos God Slaanesh, the Great Enemy of the Eldar, who leeches the soul-essence of the Dark Eldar while they still live. Dark Eldar drink souls to stave off this leeching - perhaps by sating the thirst of Slaanesh, or perhaps by replenishing the essence of their own souls with that of the consumed one. Slaanesh will also consume the souls of Dark Eldar whole should they die. Dark Eldar are long-lived but not immortal; drinking souls has a rejuvenating effect that reverses aging, thus Dark Eldar need not fear falling into the clutches of Slaanesh due to death from old age, if they have a constant supply of souls. The usual source of souls are those of the many captives taken during Dark Eldar raids.
The Dark Eldar are the Warhammer 40,000 universe's equivalent of the Dark Elves of Warhammer Fantasy - much as the Eldar are the equivelent of the High Elves.