Event · M31

Isstvan III Atrocity

The Isstvan III Atrocity was the virus-bombing of loyalist Space Marines on Isstvan III, opening Horus Lupercal’s rebellion against the Emperor. It took place early in the Horus Heresy, dated to around 005.M31, in the Isstvan system, when four Space Marine Legions committed to the world’s pacification — the Sons of Horus, World Eaters, Emperor’s Children and Death Guard — were divided between those who followed Horus into treachery and those who remained loyal to Terra. Isstvan III, the system’s third planet, has remained a quarantined and uninhabitable world ever since. The pretext for the expedition was an insurrection in Isstvan III’s capital, the Choral City, where the former Imperial governor Vardus Praal had renounced Compliance and turned the population against the Imperium. Horus, together with Angron of the World Eaters, Mortarion of the Death Guard and Emperor’s Children forces under Lord Commander Eidolon, assembled a joint expedition to suppress the revolt, and loyalist and traitor companies fought side by side to retake the city; the Emperor’s Children officer Lucius is recorded as having personally slain Praal before later defecting to Horus’s cause. Only once the Choral City was secured, and the loyalist officers among the four Legions had been identified and committed to the surface under the pretence of continued pacification, did Horus set his true purpose in motion. With the loyalists isolated on the ground, the traitor fleet withdrew its own supporters to orbit and unleashed virus bombs upon the planet, an act intended to annihilate the abandoned companies outright; accounts differ on the precise toll, though many billions of the Choral City’s population and the stranded Astartes are recorded to have died in the opening attack. Survivors who endured the life-eater virus in sealed districts of the city were hunted for some three months by traitor ground forces, with Eidolon prominent in directing the killing. Among those who survived were Garviel Loken of the Sons of Horus and Saul Tarvitz of the Emperor’s Children, who worked to rally other survivors and expose the betrayal, while Nathaniel Garro of the Death Guard escaped aboard the frigate Eisenstein together with the veteran Iacton Qruze to carry word of Horus’s treachery to Terra, their passage endangered by the traitor Calas Typhon. Accounts differ on how far officers such as Tarik Torgaddon and Horus Aximand, known as Little Horus, were already committed to the Warmaster’s cause at this stage. The Atrocity marked the point at which Horus abandoned concealment, setting the course for the wider Horus Heresy and the greater massacre that followed at the Isstvan V Dropsite Massacre. It is remembered alongside the Silencing of Isstvan Extremis, fought earlier in the same campaign, and the Flight of the Eisenstein, which carried its warning onward toward Terra; Isstvan III itself, its Choral City left in ruin, has been maintained under quarantine ever since as the site of the betrayal’s opening act. Calas Typhon, First Captain of the Death Guard, helped orchestrate the virus-bombing of Isstvan III, remaining with the fleet rather than taking part on the surface. Because Nathaniel Garro could not join the loyalists on the surface, Typhon assigned him to the frigate Eisenstein alongside Ignatius Grulgor, and privately ordered Grulgor to deal with Garro. When the Eisenstein left orbit, Typhon pursued it in the Terminus Est, damaged the frigate, but did not stop it from reaching the warp. Mortarion led the Death Guard in person through the western trench and bunker line of the Choral City on Isstvan III before the virus-bombing. Lucius, a captain of the Emperor’s Children, led his Legion’s half of the assault on the Precentor’s Palace in the Choral City, killing Vardus Praal in the course of the attack; Squad Nasicae fought at his side throughout. Before the virus-bombing of the city, Tarvitz urged him to relay a warning of the attack to the loyalist defenders. After the bombardment, Lucius fought for a time among the loyalist survivors, then killed the traitor Chaplain Charmosian and used Charmosian’s helmet communicator to contact Eidolon. He subsequently killed the loyalists placed under his command, among them the men of Sergeant Kaitheron, and deceived and killed Captain Solomon Demeter. Tarvitz defeated him in a duel, and he escaped to the traitor lines; of Squad Nasicae, three members survived the campaign. Fulgrim, primarch of the Emperor’s Children, took part in the Isstvan III campaign. Before the main landings he was sent to sound Ferrus Manus of the Iron Hands, and Lord Commander Eidolon commanded the Legion’s companies on the surface during the assault on the Choral City; Fulgrim returned to the traitor fleet before the withdrawal from the system, bringing news that Ferrus had refused Horus’s cause. The Emperor’s Children under Fulgrim purged those of their own Legion who remained loyal among the companies abandoned on the surface after the virus-bombing. After the Battle of Isstvan III, Fulgrim came into possession of the Anathame, the weapon earlier associated with Eugen Temba and the wounding of Horus on Davin’s moon. Khârn, captain of the World Eaters 8th Assault Company, fought on the surface during the infighting and met Garviel Loken, a loyalist captain of Horus’s own Legion who was resolved to oppose the Warmaster, in close combat. Loken threw him against the dozer blade of a nearby vehicle, which impaled his chest, and he was believed killed in action. A warrior of his company, Skane, found his body, and his friend Apothecary Kargos saved his life.

See where Isstvan III Atrocity occurred on the galactic map during the Horus Heresy.

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