Shortly after the battle for the Choral City had turned in the Space Marines' favour, all communications with the orbiting Imperial fleet had ceased and the Titans of the Legio Mortis began a silent and implacable withdrawal beyond the city walls into the empty plains beyond. With inexorable slowness the great Battleships and Battle Barges of the combined fleet -- Vengeful Spirit, Firebird, Andronius, Killing Star, Indomitable Will, Gauntlet of Spite, Warchild, and Conqueror -- descended into low orbit and began saturation bombardment of the planet. Orbital bomb racks bloomed and tiered decks of Macro-Cannon batteries unleashed torrent after torrent of shells downward into Istvaan III's lower atmosphere. The deadly cargo carried by these munitions was the Life-eater Virus, one of the most terrible Exterminatus-class weapons in the Imperial arsenal, and a weapon whose use only the Warmaster and the Emperor could have commanded. Dispersed by concussive charge from a thousand exploding munitions, the Life-Eater Virus began its work in moments, infecting and destroying, a rapidly-spewing necrophage which turned every living thing it came into contact with into sickening liquid rot. Human flesh sloughed from bones and screams were cut off as victims drowned in the decaying fluids of their own lungs. Plant life withered and melted into brown-black sludge as if time had turned it to the aged blight of years in mere minutes. Black gales of rot and corpse vapour moaned through the concrete and steel canyons of the cities, sounding the death knell of eight billion souls. Beyond the graveyard cityscapes, the wide plains and jungles collapsed into cankerous decay, while the oceans turned to greenish sludge so that to the warships of Horus' fleet high above it appeared that cancerous stains rapidly spread across the surface of the planet until it hung like rotten fruit against the blackness of the void.
As night fell, the debris and dust thrown into the upper atmosphere by the planetary firestorm began to descend, the bleeding heat of the dying world birthing fresh continent-spanning rainstorms which ravaged the tortured air of Istvaan III and closed over the Choral City with renewed force. Hurricane-force winds drove great torrents of still-hot ash through the ruined canyons of city-blocks and tombstone structures peopled with the dead, while thunderous volleys of lightning played havoc with Vox-transmissions and Auspex surveys. Flight was virtually impossible and the surface all but cut off from the fleet. While the first stage of the plan, the separation of the Loyalists, had proved successful, the second phase was approaching unmitigated disaster, a fact that must have been readily apparent to the master-tactician that Horus undoubtedly was. Yet even masters of their craft, post-human and human alike, are subject to the whims of chance and action. The virus-bombing of Istvaan III had ultimately done nothing more than slaughter billions of innocent bystanders and turn Istvaan III into a hellish warzone of rubble, ash and storm which massively advantaged the defender while it vexed the attacker in turn.