I was thinking about the possible origins of the Mandrakes and the Haemonculus' and it occurred to me that both groups may be what remains of the Dark Kin's psychic ability, excluding the occasional aberrant rogue witch that might pop up from time to time.
It seems to me that the whole Power From Pain "technology", including all the soul-eating and souls-as-power-sources might have it's origins in the first generation of Haemonculai being psykers, possibly Spiritseers? It's suggested that Haemonculai are descended from the rulers of the pre-fall empire but they're clearly not of the Eldar aristocracy. So that suggests that some other class was ruling them. given the Craftworlds are lead by the Farseers it makes sense that WArlocks ruled in the old empire. That would suggest that the Haemonculai ancestors were Warlocks. It's possible that the arts of the Haemonculai proper and those of the Craftworld Spiritseers are both branch descendants of pre-fall Eldar Necromancy. Maybe the Haemonculai pursued it to it's ends (hurrying the fall in the process) whilst the moderate/cautious Eldar put a ban on it (only for it's moderate applications to be rediscovered some time later by the Iyanden).
With the coming of The Fall and the effective imprisonment of the Dark Kin in the Webway the soon-to-be Haemonculai had to put a stop to all psychic ability in Commorragh. But they wont have done this until they had transformed their great psychic knowledge into practical technology. Given how their Pain Engines run on torture and anguish it wouldn't surprise me if some conclave of soon-to-be Haemonculai, the second generation, banded together to bind their Warlock masters into the machines that would fuel their Power From Pain machines. What a fitting and delicious end for the chief architects of The Fall! Maybe the Haemonculai today are the Eldar with the greatest latent psychic ability? Maybe they use that sensitivity to the those energies to help them power the Engines of Pain so that they never have to flex their psychic muscles at all. It's very probable the Haemonculai themselves (even Vect himself) is unaware of this vestigial psychic link?
As for the Mandrakes, it's a personal theory of mine that they're latent psykers who have found their way into the darkest recesses of the Dark City. The vast majority of the souls that disappear in the lowest reaches of the city and those who are mysteriously dragged into shadows are probably suffering some horrible fate deep in the darkest reaches of the shadow realm. But maybe some, those with latent psychic potential are spared somehow, perhaps protected from that fate? What if the shadow bonds with them, enters their body and mind? Maybe the shadow is semi-sentient and it's seeking out these individuals for this purpose? It would help answer the question of why the Mandrakes hunger so much and why their weird baleblast abilities are heightened by killing sentients. Seems to me that baleblast is a manifestation of psychic abiity and the Mandrakes are clearly otherworldly. Might The Shadow (for lack of a better name) be seeking hosts to spread it's reach and feed it's hunger more efficiently? The Mandrakes psychic potential allow them to become "something else" when The Shadow claims them. I doubt this thing is of demon origin. It's probably something far more alien.