The Isstvan III Atrocity

The Isstvan III Atrocity was the moment Horus's rebellion cast aside its mask and revealed its true and monstrous face. Before he could march on Terra, the Warmaster had first to be certain of his own Legions, for within the Sons of Horus, World Eaters, Death Guard and Emperor's Children there remained many thousands of warriors who would never willingly raise a hand against the Emperor.

Horus resolved to murder them all in a single stroke. He gathered the suspected loyalists into a ground assault to reclaim the rebel-held world of Isstvan III, and once they were committed to the surface and beyond recall, he ordered his fleet to bombard their positions with life-eater virus bombs. Whole continents dissolved into rot and ash in an instant, and billions of lives were snuffed out in the span of a few heartbeats.

Yet the massacre did not unfold as the Warmaster had planned. Warned by instinct and by a handful of clear-eyed commanders, thousands of loyalist Astartes survived the initial strike, sheltered in bunkers, ruins and the deep places of the shattered world. Led by warriors such as Garviel Loken and Nathaniel Garro, they waged a doomed guerrilla war against their own traitor brothers amid the poisoned wastes, buying time with their lives so that word of the great betrayal might somehow reach Terra. Their doomed defiance turned what should have been a swift execution into a grinding campaign that cost the traitors dearly in time and blood.

The ground war dragged on for months as the traitors ground the last defenders down street by street and bunker by bunker. The atrocity marked the true opening of the civil war, and it is set within the wider tragedy chronicled in the Isstvan III Atrocity and the Age of Darkness.