"You should be grateful. Once I remove your skin, you will feel so much cooler. And when I'm done with you, you shall serve a greater purpose."
- – Urien Rakarth (to a random captive Space Marine he was experimenting on in the 3rd edition Dark Eldar Codex)
The... thing known as Urien Rakarth is truly one of the sickest freaks within the 41st Millennium and we are talking the grimdarkness of the far future, where sick fucks like FABULOUS Bile, Slaanesh, and mad skeleton robot spider scientists exist.
He is one of the Haemonculi, those fun-loving Dark Eldar who enjoy nothing more than exploring up to and beyond the limits of your pain threshold, just for the sake of it (and their own perverted amusement/curiosity/boredom). Urien is amongst the oldest of his brethren and a genius in his field, regardless of if this field were torture, flesh-sculpting or anatomical body mutilation. The fact that he is a genius, but has hardly ever produced anything of note, says worrying things about the skills of the Haemonculi. He no longer cares about the Dark Eldar's politics; he just wants to be as depraved as possible now. He is utterly mad, often wandering around his laboratories gibbering crazily to himself about finding more experiments, sometimes even while already torturing someone. In one White Dwarf battle report (after a Dark Eldar victory over Craftworld Eldar), there's a piece of fluff where Urien's thrilled at the chance to eat a Farseer's bones because he likes the taste and eating them can grant visions. So not only is he the only canon example of an Eldar cannibal (souls may not count, it's because he will eat other Eldar), he's one of the few Dark Eldar who'll violate one of Commorragh's very few laws.
How black a soul is he? He laughs at all the examples of 'horror' movies mankind makes. He shakes his head with paternal tolerance at the idea of a human centipede, simply asks if that is all, and suggests further ways to add to it. Anything taboo he yawns at and says been there, done that. There are Dark Eldar and then there is Urien Rakarth.
So old that he was a first-hand witness of the Fall of the Eldar, he has been reborn into new bodies countless times and actually now comes to relish the feeling of dying each time, just another experience to the crazed scientist. However, something has gone wrong in his reincarnating process, or it could be his utterly twisted soul is now affecting his physical form, but he is now as much of a monster in body as he is in mind and spirit. Every time he regenerates, his new body has a vestigial body part replicated from his previous one, with each death adding a new part. His spine is a twisting compound wreck that leaves his posture constantly sloping, and he has multiple limbs, some decaying or dead, poking out of every angle of his back. His face itself is now held onto his skull with pieces of leathery flesh. In a way he now resembles a puppet, his broken and yet functioning body moving at his commands despite it not being possible in any realistic way.
Despite his madness he is still as expert in his craft as ever, able to create constructs that outshine his fellows, and he's considered arguably the most skilled of Dark Eldar Haemonculi. The lore heavily implies he's the one who came up with the Dark Eldar's ability to physically regenerate by feeding on suffering, and if even a single one of Rakarth's bones is found (right down to a single digit bone from one of his vestigial hands), he can regenerate from that. His über Grotesques are stronger, tougher, and, although it makes no sense considering what he must do to them, are saner and more able to follow commands. It is no surprise then he is still regarded as the most senior of the haemonculi masters, and still leads his own coven, the Prophets of Flesh.
It is believed Slaanesh is desperately trying to scout out his talent for a position at his palace as head torturer/plastic surgeon to the daemonettes. So far Urien has declined these most generous offers but like with all Eldar, Slaanesh will eventually get his/her/its man. Assuming Urien is ever going to die for good, which we may safely bet would never happen (though the Imperium may have one way of doing it).
Before the Gathering Storm he orchestrated a series of raids to capture millions of people and put them in stasis so he can take torture and feed off people if anything bad happens and the Dark Eldar can't do raids anymore. Since the stirring of Ynnead Urien has, like all the Haemonculi, made clear their opposition and fear of this new God and its followers, and remains firmly aligned to Asdrubael Vect in Commorragh, opposing any sort of cooperation or alliance with the Ynnari whilst, simultaneously, being interested in their abilities to cheat death. He also planned to try and take control of the Yncarne to get complete mastery over death.
When Vect was killed, Urien provided one of his finest performances for the funeral. However, at the height of it, he had his creations slaughter all the other Archons, and their combined suffering rejuvenated Vect and brought him back to life. Urien Rakarth proceeded to resurrect every Archon there; the Archons loyal to Vect were brought back as they were while the disloyal ones were turned into monsters subservient to Vect.
Like most Dark Eldar characters he's done very little of note.