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. Thanks to Tarvitz's warning, a few hundred loyal Space Marines managed to survive inside airtight bunkers and bomb shelters. Captain Saul Tarvitz of the Emperor's Children, working on a hunch, had managed to remain in the fleet, where he discovered the planned bombardment and commandeered a Thunderhawk to save his betrayed brethren.

Not using the traditional Stormbirds and cutting their communication lines to the fleet, Horus tried to ensure that none of them would survive the massive virus bombardment. The planned first wave, all suspected loyalists, deployed by Drop pods upon the Choral City, the capital of the planet. Horus and these Primarchs had fallen to Chaos and took this opportunity to purge all suspected loyalists from their Legions.

Under his command were four Space Marine Legions his own Sons of Horus, the Emperor's Children of Fulgrim, Mortarion and his Death Guard, and the World Eaters led by Angron. The Emperor of Mankind and the Council, ignorant of the change in Horus, charged the Warmaster with crushing this rebellion. Baron Vardus Praal, Imperial governor of the Isstvan system, eventually rose in revolt against the Imperium, purging Imperial elements for as long as six years before word of the rebellion reached the Council of Terra.

Horus heresy book 3