Home to some 41 billion Imperial souls, Belatis' main exports into the Imperium at large are twofold: manpower from the hive worlds of Belatis I, II and the feral world Polidia, and promethium from the frozen deathworld of Cestelor. Reclaimed fairly early during the early years of the Crusade by the then-named Luna Wolves, the worlds of the Belatis system were rapidly turned over to contributions for the nascent Imperium's galaxy spanning war. The system's prodigious population was folded into multiple regiments of the Imperial Army, and it's promethium supplies were fed to the hungry ships of Expedition Fleets from the recovered Belatis Station.
With so many mortal minds to turn towards efficient contribution of resources to the Great Crusade, the system masters of Belatis soon found the problem of control coming to their tables. It was one thing to tell billions of men and women that it was their duty and honor to give their lives fighting amongst the stars for humanity's glory. It was quite another to actually make them believe it. The Mechanicum representatives on the system capital of Belatis IV had the answer. Their magi had uncovered a number of archeotech since Belatis' compliance. Among them was a series of neurological implants and immersion devices collectively referred to as "Serebrium". Not entirely unlike the hypno-conditioning used to create the Legionnes Astartes, Serebrium allowed the user unprecedented control over their subjects. Regiments marching out of Belatis' mustering fields did not flinch, complain or break ranks.
As it passed, Cerebrium was what drew the eye Warmaster upon the outbreak of his rebellion. While the billions strong regiments of Belatishi Auxiliaries would be a welcome boon, it was the arcane secrets of Serebrium that was the true prize. With the archeotech in his grasp, Horus' generals would be able to turn over entire planetary populations into willing puppets to feed into Terra's guns. Although the system governors prepared to welcome the Warmasters emissaries with open arms, the Magi of Belatis had not yet renounced their oaths. Gathering to her the most trusted of her order, the ArchMagos of Belatis IV butchered the traitorous masters of Belatis, sealed the gates of Belatis' hives and sacrificed the last of her astropathic choir to send a distress signal into the void, crying for aid from any loyalist forces nearby.
Meanwhile, deep in the bowels of the approaching Traitor fleet, the Legions' war machine readied for battle.