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