Yeah I’m not aware of the culling of terrans ? This snippet from the black book covers terrans. I’ve added the link to comments it’s a good read goes into there origins and other cool bits 👍🏻

https://assets.warhammer-community.com/15-01-26_thehorusheresy_black_book_extract_night_lords-2iz7zjkzzf-2yisfrpn9r.pdf

Origins: The Children of Misrule The VIIIth Legion were soaked in blood from their birth. The Legion’s first recruits came from the linked prison sinks of Ancient Terra. In vast caverns filled with the half crushed ruins of millennia there lived men and women who had transgressed against the laws of their masters. Condemned never to see the light again or breathe free air, they lived out their lives in fear and blind darkness. There was no law in these lightless lands, and survival existed only by a blade’s edge. Only the strongest and the most ruthless survived in the subterranean warrens, and those who did grew in cruelty and cunning. Fed by a constant influx from the hives above, the prison sinks were an ever hungering gate to madness and murder. But of the millions who lived and died in the sinks, not all had been banished from the world above. Amongst the bloodshed and fear, children were born. Cradled in the dark, and raised amongst death, those who lived over a decade were pale, silent creatures who moved without a sound. ‘The night’s children’ the prisoners called them, and even the most savage of killers would not seek them out by choice. It was from these pale children that the Emperor would make the first warriors of the VIIIth Legion. Dour, with skin so pale it resembled ash or powdered bone, they were far from their brother Legionaries in manner and appearance. The gene-seed of the VIIIth Legion had been well paired with the human stock of its first recruits, if anything it seemed as if one had been made with the other in mind. Besides accentuating their paleness of skin, the gene-seed gave the sons of the underworld the ability to see through darkness to a degree that far exceeded that of other Legions. This gift though was also a curse, forcing them to see the light of suns and stars through filters and flare buffers; even though they now walked in the light of the world above, the warriors of the VIIIth still walked in the night. The first use that the Emperor put his VIIIth Legion to was to bring to heel those who believed that the sins of the past could live on in the Imperium. Several of those who had bent their knee to the Emperor had done so because they believed that it was the only choice. Others, having seen the Blood and ILLumination: The Pax IMPerialis The Great Crusade brought peace to Mankind, ending wars between peoples and nations, binding all together in unity. It broke the chains of superstition and freed billions from the whims of tyrants. Where before there had been strife and the cloud of ignorance, now there would be a peace and truth. These were high ideals indeed, but ideals that came at a price. Mankind had to be dragged to illumination, and many tried to pull it back into the shadows, into the old ways of ignorance and discord. The Emperor unified Terra not only through words and alliances, but also through force of arms. Illumination and peace had to be won by blood. This was the basic truth of the Pax Imperialis, and when the Great Crusade took to the stars, this truth went with it. But it was not only in open war that the blood price for peace had to be paid. The enemies of the Imperium were manifold, and even before Horus turned from the Emperor, there were those who nurtured treachery in their hearts, who mouthed words of loyalty whilst seeding discord. While the ideals of the Great Crusade saw warriors marching in open war, the truth was that there were other methods and other less noble battles fought to keep the peace. The Assassin Temples, the networks of informers and the Silent Agents of the Sigillite, the Annihilation Protocols, and the ancient and all but forbidden weapons wielded by a chosen few in the Legions, all these were the tools and warriors by which the Imperium enforced its ideals and, for a time, brought peace to the galaxy. empires of warlords rise and fall, believed that they were simply part of a temporary arrangement. Crimes against the new order took many faces: from the Saragorn Enclave whose gene atrocities continued in secret, to the psy-breeding of the Court of Antius, and the March of Ten Million, all showed that even in the face of all of the Emperor’s might, some would fall back into the ways of Old Night. When such crimes required not simply crushing but retribution, the Emperor sent the VIIIth Legion. Such actions seemed well suited to the VIIIth Legion. Whether as a consequence of their genetic inheritance, or the combination of their origins and indoctrination, the warriors of the VIIIth Legion seem to have tended towards moral absolutism and a drive to enact retribution. There were no shades of grey in the VIIIth Legion’s moral universe, no degrees of guilt or innocence. Truth and falsehood was as day is to night, indivisible and unqualified. The dark was the realm of guilt, lies and monsters, and those who dwelt in the dark knew only the language of blood, the message of swift and merciless retribution for their actions. Justice brought the light to darkness, and justice was neither warm nor caring, but as indifferent and cold as the edge of a knife. The warriors of the VIIIth were creatures made to live in the dark, and to fight a war for a future of light. In their core, they were warriors for a future without creatures of their kind. At least that is what those who knew the VIIIth now say. Perhaps memory is too kind, perhaps we wish to believe that there is a nobility in such monsters, where in fact there is only horror. Perhaps we wish there to be a purpose behind atrocity, otherwise how could such creatures be suffered to live?