Baron '''Vardus Praal''', Imperial governor of the Isstvan system, eventually rose in revolt against the Imperium. The [[Emperor of Mankind]] and the [[Council of Terra]], ignorant of the change in Horus, charged the Warmaster with crushing this rebellion. Under his command were four [[Space Marine Legion]]s; his own [[Sons of Horus]], the [[Emperor's Children]] of [[Fulgrim]], [[Mortarion]] and his [[Death Guard]], and the [[World Eaters]] led by [[Angron]]. Horus and these Primarchs had fallen to Chaos and took this opportunity to purge all suspected Loyalists from their Legions.
All suspected Loyalists were deployed with [[Drop pods]] upon '''Choral City''', the capital of the planet. Not using the traditional [[Stormbird]]s, and cutting their communication lines to the fleet, Horus tried to ensure that none of them would survive. Afterwards, he ordered a massive [[Virus bombs|Virus bombardment]] upon the planet. However, Captain [[Saul Tarvitz]] of the Emperor's Children, working on a hunch, had managed to remain in the fleet. To his horror he discovered the planned bombardment and managed to seize a Thunderhawk to save his betrayed brethren. The bombardment caused 12 billion deaths among the inhabitants{{Fn|2}}{{Fn|3}}, every Loyalist that couldn't reach shelter in time (World Eaters suffering the greatest losses) and the utter consumption of all organic matter upon the planet. Thanks to Tarvitz's warning a few hundred loyal Space Marines managed to survive inside airtight bunkers and bomb shelters. As the putrid gases resulting from the rapidly decaying organic matter filled the atmosphere, Horus ignited a firestorm on the planet which melted the hives and razed the cities. The psychic death scream of the 12 billion is said to have been louder than the [[Astronomican]]{{Fn|2}}{{Fn|3}}.
Afterwards, he ordered a massive [[Virus bombs|Virus bombardment]] upon the planet. However, Captain [[Saul Tarvitz]] of the Emperor's Children, working on a hunch, had managed to remain in the fleet. To his horror he discovered the planned bombardment and managed to seize a Thunderhawk to save his betrayed brethren. The bombardment caused 12 billion deaths among the inhabitants{{Fn|2}}{{Fn|3}}, every Loyalist that couldn't reach shelter in time (World Eaters suffering the greatest losses) and the utter consumption of all organic matter upon the planet. Thanks to Tarvitz's warning a few hundred loyal Space Marines managed to survive inside airtight bunkers and bomb shelters. As the putrid gases resulting from the rapidly decaying organic matter filled the atmosphere, Horus ignited a firestorm on the planet which melted the hives and razed the cities. The psychic death scream of the 12 billion is said to have been louder than the [[Astronomican]]{{Fn|2}}{{Fn|3}}. Despite this, many Loyalists of the four Legions managed to survive, and an enraged Angron, acting against Horus' orders, led his Legion to the surface to slaughter the survivors. Horus, trying to salvage the situation, sent detachments of all his Legions to slay the Loyalists. The Loyalists, fighting tenaciously, turned the planned slaughter into a full-scale battle. Under the command of Captains [[Garviel Loken]] and [[Tarik Torgaddon]] of the Sons of Horus (redubbed the Luna Wolves), Saul Tarvitz of the Emperor's Children, and [[Ehrlen]] of the World Eaters, the Loyalists were able to mount a surprising defence in the ruins of the capital city. Ultimately, the Loyalists were completely crushed, in part to [[Lucius (Emperor's Children)|Lucius']] betrayal and the orbital bombardment of the Choral City, but made Horus pay dearly. The battle lasted for three months and inflicted huge losses upon the Traitors. It was a Pyrrhic Victory at best. While the battle raged upon the planet, a handful of Loyalists led by Captain [[Nathaniel Garro]] of the Death Guard managed to commandeer a vessel, the frigate [[Eisenstein]]. The Eisenstein quickly fled into the [[Warp]], and Garro managed to warn the [[Emperor]] of Horus' betrayal. Horus prepared a trap on the planet [[Isstvan V]], which was much more successful.
==Isstvan and Istvaan==
