Realm of Chaos: The Lost and the Damned (1990), p178 — 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 Chapter 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, they were tempted to lead their Space Marine Chapters 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. By the same token the Marine Chapters that they led also turned slowly but inevitably 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 he struggled to choke the Emperor's life out of him.

Horus's fall came as a great shock to the Emperor. For a vital month the Emperor hesitated, stunned by the extent of Horus's treachery, unable to believe that his friend and general was really gathering forces against him. When the war finally broke out it was savage and bloody. Marine fought Marine as the rival factions battled for supremacy.