Codex: Chaos (1996), p8-9 — The Horus Heresy

The forces of Chaos were not quite so easily beaten. They whispered to the Primarchs from the warp, disturbing their dreams with promises of power, appealing to their pride, their martial prowess, and their courage. No single Primarch was wholly resistant to these unspoken temptations. The character of each was sorely tested, and fully half of them failed that test. So subtle was their temptation that they never even suspected how their own loyalties were changing.

For example, Mortarion Primarch of the Death Guard Legion fully believed that he was the herald of a new age of justice. Angron of the World Eaters genuinely thought that he alone could save humanity from destruction. Horus too, the greatest Primarch of all, was convinced of the virtue of the martial ideals for which he fought.

By appealing to their virtue and courage, the Primarchs were tempted to lead their Space Marine Legions against the Emperor. Initially, even the Primarchs had little idea that they had fallen to Chaos, but when they rebelled their good intentions gradually fell away as Chaos saturated their souls. The Space Marine Legions that they led also turned slowly but inevitably to Chaos. The corrupting influence of Chaos soon spread to the Imperial Guard and Adeptus Mechanicus forces, including the Titan Legions and the Legio Cybernetica. From there the rot spread further into the Imperium itself. Over half of the Adeptus Mechanicus alone were ready to join an Empire dedicated to Chaos.

The leader of the rebellion was the Warmaster Horus, the greatest and most trusted Primarch of all. He had stood by the Emperor's side throughout the long years of the Great Crusade. They had fought back-to-back at the siege of Reillis when the Emperor saved Horus's life. On the battlefield of Gorro, Horus had repaid the debt by hacking the arm from a frenzied Ork as it struggled to choke the Emperor's life out of him. The Emperor had entrusted Horus with leading the crusades along the Eastern Fringe while he returned to Terra to consolidate the rule of the vast Imperium num under his control.

In the Emperor's absence Horus's plans were just coming to fruition when the Imperial commander of Isstvan III declared the whole of the Isstvan system an independent principality. The Emperor, ignorant of the change in the Warmaster, ordered Horus to pacify the system. Horus chose to do so by virus bombing Isstvan III from orbit. The voracious life-eater virus slew every living thing on Isstvan III in a matter of minutes; twelve billion souls died with a death scream which pulsed louder than the Astronomicon. Whole continents and hive cities were charred to ash as the mass of oxygen released by the instant rotting of all organic material on the planet burned in the atmosphere and covered the world in a gigantic firestorm which raged for days. Before the last fires were out Horus despatched the Titans of the Legio Mortis onto the planet's surface to root out any who had survived in protective shelters or underground bunkers.

During the bombardment a handful of Space Marines still loyal to the Emperor seized control of the Frigate Eisenstein. They had discovered the taint of Chaos spreading through Horus's command and as the Warmaster withdrew to Isstvan V to marshal his forces the loyalists fled into warp space to warn the Imperium.