Eidolon - Istvaan III Atrocity: Eidolon's descent into the clutches of Chaos only deepened during the Battle of Istvaan III after Horus betrayed the Loyalists within his own Traitor Legions to cleanse his troops of any who remained beholden to the will of the Emperor of Mankind. The Warmaster ordered the Loyalists' contingents from the the Sons of Horus, World Eaters, Death Guard and the Emperor's Children to begin the planetary assault. While these forces were busy laying siege to Choral City, the seat of Istvaan III's government, the Warmaster callously ordered the orbital bombardment of the planet. Fortunately, the Loyalist Captain Saul Tarvitz of the Emperor's Children was aboard the Strike Cruiser Andronius and had discovered the plot to wipe out the Loyalist Astartes of the Traitor Legions. He was able, with help from Battle-Captain Nathaniel Garro of the Death Guard who was in command of the Death Guard Frigate Eisenstein, to reach the surface of Istvaan III despite pursuit and warn the Loyalist Space Marines he could find of all four Legions of their impending doom. Those that heard or passed on Tarvitz's warning took shelter before the virus-bombs struck.The civilian population of Istvaan III received no such protection: 12 billion people died almost at once as the lethal flesh-dissolving virus called the Life-Eater carried by the bombs infected every living thing on the planet. In the aftermath of their deaths, a global fire storm raged as all of the dissolved organic matter caught fire. The Primarch of the World Eaters, Angron, realising that the virus-bombs had not been fully effective at eliminating all the Loyalists, flew into a rage and hurled himself at the planet at the head of 50 companies of World Eaters Traitor Marines. Discarding tactics and strategy, the World Eaters Traitors worked themselves into a frenzy of mindless butchery fed by their growing allegiance to the Blood God Khorne. Horus was furious with Angron for delaying his plans, but Horus sought to turn the delay into a victory and was obliged to reinforce Angron with troops from the Sons of Horus, the Death Guard, and the Emperor's Children, as the massive pall of ash hanging over the world after the Life-Eater attack precluded an immediate orbital bombardment.On Istvaan III, the remaining Loyalists, under the command of Captains Tarvitz, Garviel Loken and Tarik Torgaddon, another Loyalist member of the Sons of Horus, fought bravely against their own traitorous brethren. Yet, despite some early successes that delayed Horus' plans for three full solar months while the battle on Istvaan III played out and he waited for the atmosphere to clear, their cause was ultimately doomed by their lack of air support and Titan firepower. The few remaining Loyalists of the Emperor's Children Legion fought bravely on Istvaan III, led by Captains Saul Tarvitz and Solomon Demeter. To prove his worth and loyalty to Lord Commander Eidolon of the Emperor's Children -- and thus to his Primarch, Fulgrim -- Captain Lucius of the 13th Company of the Emperor's Children, the future Champion of Slaanesh known as Lucius the Eternal, turned against the Loyalists that he had fought beside because of his prior friendship with Saul Tarvitz.Given open access to the Loyalists' eastern flank, hundreds of Traitor Emperor's Children Legionaries, led by Lord Commander Eidolon, streamed into the Precentor's Palace from the east, on a tide of fire and blood. They fell upon the wounded first, Eidolon himself butchering those who lay waiting for an Apothecary's ministrations, taking particular relish in killing the Loyalist Emperor's Children he found there. The warriors of his Chapter swarmed through the palace around him, the defenders discovering to their horror that their flank had somehow been turned and that more and more of the Traitors were pouring into the palace. Within moments, the last battle had begun. The Loyalists turned from their defences and faced the Emperor's Children. Assault Marines' Jump Packs gunned them across ruined domes to crash into Eidolon's assault units. Heavy weapons troopers and Scout snipers amongst the ruined battlements shot down into the enemy, swapping tremendous volleys of fire across the shattered domes. It was a battle without lines or direction as the fighting spilled into the heart of the Precentor's Palace. Each Astartes became an army of his own as all order broke down and every warrior fought alone against the enemies that surrounded him.As the main Traitor attack column was storming the broken holdfast of the Precentor's Palace, in the northern and southern sectors further assaults were taking place to break the back of the Loyalist resistance and hunt down the scattered survivors in the ruins. In the subsequent final orbital bombardment of the planet by the Warmaster's fleet, it brought the resistance of the Loyalists to its brutal, bloody conclusion. Horus ordered the guns of the Vengeful Spirit turned once more upon the carcass of the Choral City, not with viral weapons but conventional warheads, and pulverised its ruins flat before leaving Istvaan III a corpse-cinder in the wake of the Traitors' progress. The bombardment of the planet triggered tectonic convulsions: the earth trembled, fissures opened in the ground, and buildings toppled even as Legion brothers fought one another under the collapsing roofs. Across Istvaan III, in ever-shrinking circles of defiance, the last Loyalists of the four Traitor Legions died under the blades of warriors who had once been their brothers. However, Eidolon had over-extended his lines and the Traitors themselves were outflanked in a final inspired attack by Tarvitz. Horus withdrew his troops after this, and completed the destruction of the Choral City with an orbital bombardment. This spelled the end for the few remaining loyalists. But they died knowing they had taken a blood price from those who had betrayed them.