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The Daemonculaba was the epitome of sickfuckery in Warhammer 40,000. It was a HOLY FUCK tier mad science project which, using stolen gene-seed, allowed for the creation of new Chaos Space Marines. It was first conceptualized by Dark Mechanicus geneticists who were sponsored by a faction of the Iron Warriors Traitor Legion, with the Warsmith Honsou overseeing it.
So what made it so fucked-up and evil?
The Daemonculaba process began by rounding up some human females as slaves. Once corralled, they were shackled naked within iron cages and force-fed nutrients causing their bodies to widen and bloat to unfathomably grotesque proportions, nearly making them resemble your mother. Your mom aside, none of this probably would have even been possible were they not in the Eye of Terror. Next, absolutely fucking mad scientists called Savage Morticans (former members of the Adeptus Mechanicus' biological division corrupted by Chaos) utilized surgical and chemical techniques as well as Chaos magicks to radically alter the slaves' insides and embed them with stolen gene-seed, thus completing the process of turning these women into what were known as the Daemonculaba. These living, agonized 'birthing-wombs' were thus readied for "impregnation". This process involved sealing a captive adolescent human male within the human birthing chamber -- by performing what was essentially a 'reverse c-section.'
Left to gestate like a cocoon, the candidate's body was slowly infused with the gene seed and corrupted by the powers of Chaos. Some days later, the candidate is reborn -- without any skin. Provided they weren't already dead, the candidate was then inspected to see if he had the physical match and worth to be a Chaos Space Marine. While this hothouse method could provide Honsou's Iron Warriors with a recruitment rate short of enslaving an entire world for the purpose of churning out recruits, it had a high attrition rate with side-effects like acute death and horrible mutations while still developing within the womb. If he was found to be an unsuitable candidate, his corpse or the still living candidate was literally flushed down the sewers to die in the barren wastelands surrounding the fortress laboratories, provided that he actually survived long enough to get to that point. Occasionally though, the rejected creatures survived being cast out and many of these banded together as huge, monstrous scavenger-hunters known as the Unfleshed (who happened to worship the Emperor and built an effigy of him).
However, if the candidate passed the inspection, skin would be cross-stitched onto its body and its new life as a Chaos Space Marine would begin. The skin was harvested from the flayed bodies of human slaves whose flesh had been painfully stretched to vast proportions before being removed while they were still alive to accommodate the demands for large amounts of skin.
While this system mostly utilized loyalist gene-seed (probably exclusively from Imperial Fists stock), it could also use genetic material extracted from "recycled" Iron Warrior corpses, which would then be implanted within the Daemonculaba.
As for the Daemonculaba themselves, if they happened to survive the birthing process, the cycle would days later begin anew, with them again being used as surrogate birthing-wombs...
And then Slaanesh is just looking at all this with interest, their eyes narrowed, and it says; "hmm, that's an interesting act, what do you call yourselves?" and Honsou sticks his chest out and goes; "The Aristocrats!"
It is rumored that the idea for the Daemonculaba wasn't actually Honsou's, but that only someone as evil as Honsou would ask for advice from the /d/arkest place in the galaxy and actually take it seriously.
Varro Tigurius of the Ultramarines had a really horrible nightmare about the Daemonculaba, and Marneus Calgar ordered the then recently dishonoured Uriel Ventris and Pasanius Lysane to go and destroy it. With the help of the Unfleshed, Uriel and Pasanius succeeded, but Honsou was left filled with such rage that he started gathering an army to wipe out Ultramar.
This is still roughly twice as romantic as Breaking Dawn.
YES Though it must be noted that this is only one artist's interpretation (and a loose one at that, considering an adolescent is very much removed from being a baby). And a censored one at that. If you've ever seen the movie Slither, there is what would probably be a more accurate depiction of the Daemonculaba there. Except with alien worms. Emperor have mercy upon us all.