Dark Eldar are a Xenos race in the Warhammer 40k universe, this race of Xenos are Eldar who are so fucked up not even those who are into the BDSM fetish would wish to partake. Some of these Xenos are anywhere between 10,000 to 35,000 years old and even some of them were still alive before they accidentally birthed a new Chaos god by the name of Slaanesh by their blood orgies and depravity that is beyond all description, logic, reasoning or understanding.
These Xenos live on Commorragh, which is essentially a massive city state in the Webway, essentially a place where nobody of sane mind would go because of how dangerous and perverse it is. The Dark Eldar have their own version of the sick twisted freak from the Emperor's Children Legion Fabius Bile, however these freaky darlings are the Homonculi, monstrous beings who delight in experimentation of the unfortunates they enslave or get their fingers on.
Take a look at these Dark Eldar, hard to believe that they were just like Eldar at one point of time.