Istvaan III

Istvaan III was a Hive World in the Istvaan System. Its destruction by a virus-bombing ordered by the Warmaster Horus and the surrounding circumstances was the first overt sign of Horus' turn to Chaos, and the first battle of the Horus Heresy.

The Battle of Istvaan III
Baron Vardus Praal, the Imperial Planetary Governor of Istvaan III and the rest of the system, had risen in revolt against the Imperium of Man after being corrupted by Slaanesh. The Emperor and the Senate of the newborn High Lords of Terra, ignorant of the change in Horus, charged the Warmaster with crushing this rebellion. Under his command were four Space Marine Legions and their Primarchs; 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 already fallen to Chaos and took this opportunity to purge all the remaining suspected Loyalists from their Legions.

All of the suspected Loyalists in the Traitor Legions were deployed with Drop Pods upon Choral City, the capital city of Istvaan III. Not allowing the Loyalists to use the traditional Stormbirds, and cutting their communication lines to the fleet after their deployment, Horus tried to ensure that none of the Loyalists would survive. After the landing, as the battle still raged, Horus ordered a massive virus bombardment upon the planet. However, Captain Saul Tarvitz of the Emperor's Children, working on a terrible hunch, had managed to remain in the fleet. To his horror, he discovered the planned bombardment and managed to seize a Thunderhawk and make his way to the planet's surface to save his betrayed brethren. The virus bombardment caused 12 billion deaths among the inhabitants of Istvaan III, and every Loyalist Space Marine that couldn't reach shelter in time also fell to the virus, which utterly consumed all organic matter upon the planet. Thanks to Tarvitz's warning, a few hundred Loyalist Space Marines managed to survive the bombardment inside airtight bunkers, buildings and bomb shelters. As the putrid gases, mostly methane, resulting from the rapidly decaying organic matter filled the world's atmosphere, Horus' battle barge fired upon the world and ignited a firestorm on the planet which melted its hives and razed the cities to the ground. The psychic death scream of the 12 billion people of Istvaan III is said to have been brighter than the beacon of the Astronomican for the psychically aware.

Despite this massive destruction, many Loyalists of the four Traitor Legions managed to survive the global inferno, 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 Traitor 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, 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 world's capital city. Ultimately, the Loyalists were completely crushed, in part due to the betrayal of Captain Lucius of the Emperor's Children and the orbital bombardment of the ruins of the Choral City, but the Loyalists' actions made Horus and the Traitor Legions pay dearly for their victory and slowed down the advancement of their ultimate plans. The battle lasted for three months and inflicted huge losses upon the Traitor Legions. It was a Pyrrhic Victory at best and one that would go a long way towards ultimately ensuring the Traitor Legions' defeat.

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 next prepared a trap for the Loyalist Legions he knew would be sent to confront him by the Emperor on the planet Istvaan V, which proved far more successful.

Alternate Spelling
The star system and planet's name is alternatively spelled Isstvan.