So a fellow Fa/tg/uy and I were recently dicussing the concept of Vampire Counts and how the could be implemented into Veeky Forums, and here's what we've come up with so far
Once upon a time there was an Imperial Sector full up with Nobles of all sorts who had existed within a strictly regimented eugenically based state before the IOM came and nommed them up.
Over time their incestuous ways [Insert webbed toe joke] and general dicking around with warpy drugs resulted in the whole nobility of the system falling under a simple very, very fucked up bloodline.
Along with most of the physical deformities and embarassment that comes with inbreeding to shit, the chaps had also somehow picked up an inherited birth defect that effectively acted as Sickle Cell on steroids: they could no longer produce any blood cells of their own and autosanguine was violently rejected by their body.
Fearing the extinction of their precious bloodline, the clan's members quickly set upon a solution: use the vast numbers of serfs accumulated on their planet sized feudal estates as a massive blood bank.
As things grew even more desperate, they skipped the blood bag entirely and went straight to the source, adopting cybernetic fangs that drew blood from their victims directly into their veins.
Of course, this rampant mutanting and mysterious dissapearance of large swathes of their peasants didn't go unnoticed, and the Imperium quickly dropped the hammer on the chumps.
Said chumps, however, due to the Counts unification under a certain Von Carstein and the mutation's secondary effects of spawning fucktons of really powerful Psykers with the ability to, albiet inneficiently, navigate the Warp (Some Navis Nobilite blood must have slipped in there somehow) and the Imperium's more pressing matters on other fronts, the bastards managed to hold off and survive.
Now, isolated and growing ever more twisted, the Transylv'an sector of the Vampyre Counts descends into a black mirror of the IOM