The growing disillusionment that began to show itself during bitter and pointless battles such as the [[Sak'trada Deeps Campaign]] would eventually translate into a tragic explosion of despair and rage as the Iron Warriors learned that their own homeworld, [[Olympia]], had revolted against [[Imperium|Imperial rule]]. Enraged, [[Perturabo]] drew his [[Space Marine Legion|Legion]] away from the extermination campaign they were waging upon the [[Hrud]] and led them homewards, falling upon the world with no mercy. The planet was battered into submission, and he gave orders that the cities were to enact decimation, killing one in every ten of their own or face extermination and enslavement. Cities burned, and over five million inhabitants were killed. Some Iron Warriors refused to take part in the genocide, and they too were struck down.{{Fn|25a}} In the aftermath, the legion Legion as a whole seemed aghast at their actions, aware that they had committed an unforgivable atrocity.{{Fn|1c}}{{Fn|1d}}

However, before word of their actions could reach the other arms of the [[Imperium]], the Iron Warriors once again received orders to move into action. This time it was to move to the [[Isstvan system]]. While the Iron Warriors had been engaged in suppressing the rebellion on [[Olympia]], the [[Galaxy]] had turned upside down: [[Horus]] had rebelled against the [[Emperor]], [[Fulgrim]], [[Angron]] and [[Mortarion]] had joined him, and the [[Space Wolves]] had slaughtered the [[Thousand Sons]] on [[Prospero]]. If the Iron Warriors had committed an outrage on Olympia, then their brother [[Space Marine Legion|Legions]] legions seemed determined to outdo them; the [[Horus Heresy]] had begun.{{Fn|1d}} Horus promised Perturabo not only forgiveness for his genocide on Olympia but even commended him for it. So Perturabo soon swore a secret oath of loyalty to the Warmaster.{{Fn|1c}} Upon being informed of the destruction of Olympia, some Iron Warrior units broke away from their Legion, subsequently joining the Loyalists in the upcoming civil war.{{Fn|88}}