Event · M31
Isstvan III Atrocity
The Isstvan III Atrocity was the attack in which four Legions attacked their own loyalists on Isstvan III, revealing Horus’s open rebellion. The event took place on the world of Isstvan III, which has remained under quarantine since. The four Legions used virus bombs against their own loyalists and ignited the atmosphere. They then spent three months hunting survivors through the Choral City. Twelve billion people died in the opening attack. Eidolon led the Emperor’s Children in the killing. The Eisenstein escaped the destruction in order to warn Terra, and Iacton Qruze left aboard the vessel for that purpose. The engagement marked the point at which Horus ceased to conceal his rebellion. Torgaddon and Horus Aximand, known as Little Horus, were associated with both sides of the betrayal. Isstvan III has since been maintained as a quarantined world.
See where Isstvan III Atrocity occurred on the galactic map during the Horus Heresy.