Emperor's Children - Istvaan III Atrocity: Before Horus openly launched his rebellion to overthrow the Emperor, an opportunity presented itself that would enable him to get rid of the Loyalist elements within the Astartes Legions under his command. The Imperial Planetary Governor of Istvaan III, Vardus Praal, had been corrupted by the Chaos God Slaanesh whose cultists had long been active on the world even before it had been conquered by the Imperium.Praal had declared his independence from the Imperium, and had begun to practice forbidden Slaaneshi sorcery, so the Council of Terra charged Horus with the retaking of that world, primarily its capital, the Choral City. This order merely furthered Horus' plan to overthrow the Emperor. Although the four Legions under his direct command -- the Sons of Horus, World Eaters, Death Guard and the Emperor's Children -- had already turned Traitor and pledged themselves to Chaos, there were still some Loyalist elements within each of these Legions that approximated one-third of each force; many of these warriors were Terran-born Space Marines who had been directly recruited into the Astartes Legions by the Emperor Himself before being reunited with their Primarchs during the Great Crusade.Horus, under the guise of putting down the rebellion against Imperial Compliance on the world of Istvaan III, amassed his troops in the Istvaan System. Horus had a plan by which he would destroy all of the remaining Loyalist elements of the Legions under his command. After a lengthy bombardment of Istvaan III, Horus despatched all of the known Loyalist Astartes down to the planet, under the pretence of bringing it back into the Imperial fold. At the moment of victory and the capture of the Choral City, the planetary capital of Istvaan III, these Astartes were betrayed when a cascade of terrible Life-Eater virus bombs fell onto the world, launched by the Warmaster's orbiting fleet.The Loyalist Captain Saul Tarvitz of the Emperor's Children, however, 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. The psychic shock of so many deaths at one time shrieked through the Warp, briefly obscuring even the glowing beacon of the Astronomican.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.Fortunately, a contingent of Loyalists led by Battle-Captain Garro escaped Istvaan III aboard the damaged Imperial frigate Eisenstein and fled to Terra to warn the Emperor that Horus had turned Traitor. 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 months while the battle on Istvaan III played out, their cause was ultimately doomed by their lack of air support and Titan firepower. During the battle, the Sons of Horus Captains Ezekyle Abaddon and Horus Aximand were sent to confront their former Mournival brothers, Loken and Torgaddon. Horus Aximand beheaded Torgaddon, but Abaddon failed to kill Loken when the building they were in collapsed. Loken somehow survived and witnessed the final orbital bombardment of Istvaan III that ended the Loyalists' desperate defence.The few remaining Loyalists of the Emperor's Children Legion fought bravely on Istvaan III, under the command of Captains Saul Tarvitz and Solomon Demeter. To prove his worth and loyalty to Lord Commander Eidolon of the Emperor's Children -- and thus to 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.Lucius wanted to punish Tarvitz for taking command of the defence, which had incited Lucius's fierce jealousy of his fellow captain. Lucius slew many of his former comrades personally, an act for which he was then accepted back into the IIIrd Legion on the side of the Traitors. In the end, the Loyalists retreated to their last bastion of defence, only a few hundred of their number remaining. Finally, tired of the conflict, Horus ordered his men to withdraw, and then had the remains of the Choral City bombarded into dust for a final time from orbit.