The Compliance of Melchior was a joint-Imperial Compliance of the world of Melchior that was carried out by both the Luna Wolves and the Blood Angels Legiones Astartes during the latter years of the Great Crusade in the early 31st Millennium against the vile xenos designated as the Nephilim.
The world of Melchior was discovered by the forces of the Imperium during the latter years of the Great Crusade. The human population was held in thrall by the malevolent xenos species designated as the "Nephilim". Melchior served as the last bastion of these foul creatures -- hulking entities of roughly humanoid shape who were smooth like carvings of soapstone, with abstract shapes approximating arms and legs. Their dome heads emerged from their shoulders without a neck, and an array of olfactory slits and eye-spots ringed the surface of their skulls. In the light, the Nephilim looked like objects crafted of blown glass, their semi-transparent flesh glowing in the bright day. The Nephilim possessed an unhurried, careful agility like that of sea-going creatures seen through the walls of a glass tank. They moved deceptively slowly through air as if they were swimming in water, but they could move fast if they wished, darting and spinning, becoming difficult to hit. The alien giants, mocking humanity's great dream of peace and unity, had left a trail of destruction behind them that had claimed a hundred worlds before they had come to rest upon Melchior.
Sagan, the DeCora Spine, Orpheo Minoris, Beta Rigel II; each of these planets had been denuded of all Human life, populations herded into empath-chapels as big as mountains and then slowly consumed. Even their name, Nephilim, the name of the fallen seraphs, was a name taken from ancient human mythology -- that of Terra, Caliban and Barac. The true horror of it was that the Nephilim used those they preyed upon to do their soldiering for them, snaring the pliant, the lonely, the sorrowful with their ideal of an attainable godhood. They plied their victims with stories of eternal existence for the faithful, of endless sorrow for the agnostic; and they were very good at it. Perhaps the xenos really believed that what they were doing was somehow taking their victims closer to a form beyond flesh, to an afterlife in an eternal heaven-state; it did not matter. With their advanced technology they implanted bits of themselves into their thralls to further their communion. They cut their own flesh and made living masks to mark their devotees.
The Nephilim controlled minds, either through the transmitted power of their will or through the weak character of those they chose. They were an affront to the Emperor's hope for the creation of the rationalist, secular galaxy outlined by the Imperial Truth. They represented not only an offence to the purity of a precious human ideal but in their insidious cuckoo-nest displacement of those who foolishly gave them fealty. For what the aliens fed upon, what the Scout Marines of the Blood Angels and Luna Wolves had seen and reported back, were the very lives of those who cherished them. The empty chapels were piled high with stacks of desiccated corpses, bodies that had been aged years in only hours as all living essence was siphoned from them. The Primarchs of the Legions were disgusted as the true understanding of the enemy they faced was revealed.
The Nephilim fed on adulation. In the strategium of his flagship, the Vengeful Spirit, the Horus showed his brother Sanguinius, the Primarch of the Blood Angels, the plan that he had conceived to break the will of the Nephilim. Horus wanted the Blood Angels to march shoulder-to-shoulder with their cousins, cowing the aliens with the sight of an army of thousands of powerful Astartes rolling without pause to the gates of their last bastion. And then through those gates, over the battlements, not stopping, not pausing to parley or hesitate. "Like the ocean these things sprang from," Horus had said, "we will roll over the aliens, drag them down and drown them." The sheer bombast of the plan was its greatest strength, but Sanguinius had not been easily swayed to it. This blunt, brute-force approach was better suited to their more intemperate brother Primarchs, to Leman Russ or to Angron. Sanguinius felt that neither he nor Horus were so artless, so focused upon the target to the detriment of all else, but in the end he agreed.
When the time came to enact their audacious plan, Horus and his Luna Wolves met the xenos army on the sparkling white plains of the Silver Desert. Before open hostilities began, Horus attempted to parlay with the Nephilim one final time, but the overconfident xenos refused to capitulate, casually informing the humans that they could not win. Horus informed the xenos commander that it had made a grave error, for it would be the Nephilim who would fall. The Luna Wolves would be the anvil upon which the xenos would break, and the Blood Angels would be the hammer. With lightning speed, the heavens screamed as a rain of ceramite Drop Pods tore through the outer atmosphere of Melchior and fell like flaming meteors towards the Silver Desert. Falling with them were Stormbird and Thunderhawk assault gunships that turned and wheeled through the air towards the gargantuan Nephilim encampment, carrying company upon company of the IXth Legion. The speed of their assault was the key to victory; the alien invaders and their zealots had successfully been drawn out to confront the massed forces of the Luna Wolves, leaving the defences on their flanks thinned and permeable. But the xenos giants were not slow in their thinking, and the moment they understood that they had been duped, they would attempt to regroup and fortify. The Blood Angels did not allow that to happen. The Nephilim were broken and cut down, their cohesion shattered by the brutal deep strike. Caught between the brutal fury of two Space Marine Legions on Melchior's shining sands, the Nephilim were finally put to the sword.
- Fear to Tread (Novel) by James Swallow