Chaos by its very nature does not promote the traits needed to build a civilization. Much of the Lost and the Damned exist as little more than scavenging raiders. Surviving through brutality and theft, much like the Gods they serve. But like most things, this rule has an exception. In the galactic south-east exists an Empire in service to the Dark Gods. Forged by Korban the Eversacrifice, through his own cruel will and using secrets taken from the Selenar Gene-cults. The accursed realm of Ultramar Tenebrae arose as a rival to the Imperium of Mankind.
Protected by the great warp storm that covers the galactic far-east. Ultramar Tenebrae grew quickly in parallel to the Imperium. Only through the Dark God's blessing could ships safely traverse this section of the Galaxy and that fact allowed Korban to quickly claim hundreds of worlds for the Dark Gods. Ultramar Tenebrae like all domains of Chaos is marked by corrupting evil, but there, that evil takes a different form. Not the feral self-destructive mania so often seen, but a focused harsh form of malice. Industrial, practical, incredibly cruel, these are the watchwords of Macragge and its vassal worlds. Here the genetic lore of the Selenar was put to terrible use.
The ability to quickly and effectively mass-clone humans in the clutches of true evil produced many horrible things. Billions of humans lived and died as fodder for Chaos. Designed on a genetic level for heightened emotional response and fed into "factories" of worship shortly after birth. While this form of sacrifice and worship fed Chaos less than the usual offerings, the sheer quantity made up for the quality. Here Korban and his minions tapped into the unborn domain of Hashut, embracing the mind-numbing scale of suffering that he oversees. Where Slaanesh drives Chaos to new heights of excess, Hashuts influence does something worse. It makes it's evil efficient.
The stolen blood of two Primarchs, alongside the blood of Be'lakor, did not go unused. While the Primarchs and their sons might be beyond the Dark God's reach, they could still create armies. Bastard Astartes, lesser in every way except in malice. These Iskur Hordes or Chosen Warriors as they are commonly called, are created when a young child drinks from the ever-flowing chalice of corrupted blood. If they are strong enough they are twisted into mountains of cruel, violent muscle. Blessed by the Gods and empowered by flesh-sorcery stolen from the Emperors. These Warriors are mass-produced as the ultimate tool of Chaos's wrath. Forming great hordes of warriors
To live in Ultramar Tenebrae is to exist in a domain of fear, cruelty, depravity, and focused madness. Where millions of people are grown, harvested, and consumed like crops. Their suffering a fine draught for the Thirsting Gods, who reward the rulers of this domain with new blessings and foul boons. The twisted seed of the Cult of the Four Phases has found fertile soil in the five hundred worlds of Ultramar. Tended by Korban the Eversacrfice and unleashed on the wider galaxy. Raider fleets guided by dark rituals leave the Eastern Storm and raid the Ultramarches, leaving devastation and death in their wake. Engaging in an eternal battle of wits and weapons with the Steel Wardens and other Imperial Defenders.
The Storm that protects Ultramar is a product of the Firetide of the Anathema and the Wrath of the Chaos Gods clashing. Producing a great band of the immaterium that is virtually unnavigable by those not sworn to Chaos. This far from Terra, the Firetide is not as strong as found in the Eye of Terror and can be bypassed with correct timing and talent. The ability to pass into the Imperium and raid its worlds is a great boon that allows Ultramar Tenebrae to match or even surpass the other regions of the Galaxy that court the Gods' attention. Great rivalries exist between Korban's elite and their equivalents in the Eye of Terror and Maelstrom. Rivalries that are only put to relative rest when the Triumvirate of Chaos declare a Doomtide upon the Imperium. The Triumvirate of Chaos is the "leading" body of the mortal forces of the Dark Gods. Consisting of Korban the Eversacrfice, Master of Ultramar Tenebrae, Arguleon Veq, Chieftain of the Maelstrom worlds, and Vileth the Herald, Creator and King of the Eye of Terror.