Isstvan III Atrocity - Betrayal: 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 Isstvan 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.Isstvan III died. The psychic shock of so many deaths at one time shrieked through the Warp, briefly obscuring even the constant shining pulse of the Astronomican. But the destruction was not over. An entire planet’s worth of decaying matter wreathed the atmosphere of Isstvan III in a thick shawl of combustible gasses. All it took was a single burning Lance strike from the Vengeful Spirit to set the planet aflame. In a second, the air itself caught light, ripping across the landscape in a howling maelstrom of fire and noise. Entire continents were laid bare, their landscapes seared to bare rock, their decayed populations vaporised in seconds as winds of fire swept across their surfaces in a deadly gale of blazing destruction. Cities exploded as gas lines went up, blazing towers of fire whipping madly in the deadly firestorm. Nothing could survive and flesh, stone and metal were vitrified or melted in the unimaginable temperatures. Entire sprawls of buildings collapsed, the bodies of their former occupants reduced to ashen waste on the wind, palaces of marble and industrial heartlands destroyed in gigantic mushroom clouds as the storm of flame swept around Isstvan III with relentless, mindless destruction until it seemed as though the entire globe was ablaze.Those Astartes who had survived the initial viral attack by finding air-tight shelter found themselves consumed in flames as they desperately sought to find cover once more. But against this firestorm there could be no cover for those who had dared to brave the elements. By the time the echoes of the recoil had faded on the Warmaster’s flagship, billions had died and Isstvan III had become a dead planet. Horus had at last declared his hand and openly defied the Emperor, and so consigned the galaxy to civil war. He had also planned that the virus bombardment would destroy a host of Space Marines he could not trust to fight for him in the coming struggle with the Emperor, but as was soon to become apparent, his plan had failed.