Olympia - Horus Heresy: As the tragic outbreak of the Horus Heresy grew closer, Perturabo had become consumed by bitterness. He believed that he and his Iron Warriors, who were often used by the Imperium as garrison forces or as siege-breakers, had not received the glory and recognition he believed to be their due.Even worse, a deep and vicious rivalry had sprung up between Perturabo and his brother Primarch Rogal Dorn of the Imperial Fists Legion, a rivalry that only increased Perturabo's sense of grievance.After Horus' corruption by Chaos, the Warmaster used Perturabo's intense resentment to fuel his growing anger at the Emperor and the Imperium itself. In this way, Horus hoped to sway his brooding brother to the side of his planned rebellion.Some have postulated that it was Horus who, time after time, engineered events and adjusted deployments to Perturabo and the Iron Warriors' detriment. For the Iron Warriors Legion the Horus Heresy came as the culmination of a series of reversals and fell tragedies that occurred in the final years of the Great Crusade.Foremost of these was the unexpected rebellion of Olympia against Imperial rule. With the death of Dammekos, the long-lived Tyrant of Lochos and the chosen Imperial satrap of Olympia, the duplicitous and viperous politics of Olympia finally burst into infighting and insurrection.The violence and division flared up worse than ever before because of the changes the Imperium had wrought on Olympia, and the discontent grew, due to generations of the planet's finest youth having been tithed for the IVth Legion, never to return.The shocking start of the rebellion struck at the heart of the Iron Warriors Legion and its master, and could not have come at a worse time, for the IVth Legion had been engaged in the almost single-handed suppression of a major infiltration of the infamous xenos species known as the Hrud during the Sak'trada Deeps Campaign. The fighting had lasted for over a standard year.All such actions against the Hrud had proved costly both in terms of lives and the sanity of those who fought such alien nightmares, and this campaign had been no exception.Perturabo and the IVth Legion, battered by the demands of the campaign, and enraged and embarrassed by the rebellion of their homeworld, returned to Olympia. Angry beyond measure, Perturabo presented the Olympian population with a simple choice: enact decimation, selecting 1 in every 10 of their own to be executed, or face complete extermination and enslavement if they refused.When the rebels refused to bend, Perturabo and the Iron Warriors brutally purged the world city by city, overrunning the fortresses he had personally built and sparing no one who stood against him. Some Iron Warriors also refused to take part in the carnage against their own people, and they too were ultimately struck down by their brother Astartes.By the time the massacre was over, five million Olympians had been killed and the rest put into vicious slavery to the Iron Warriors. Perturabo looked on at the remains of his homeworld in cold silence.Only once the great pyres were burning to cleanse the world of the heaps of corpses created by the IVth Legion's assault did Perturabo fully realise what he had done.The Iron Warriors were no longer the saviours of the Imperium; they had been destroying the alien Hrud one moment and yet, in the next, they were committing genocide against their own people. With the cooling of Olympia's mass funeral pyres had come the realisation that nothing the Lord of Iron could ever do from that moment on could ever atone for such a worldwide genocide.Perturabo believed that his father the Emperor would never forgive him so grievous a sin, but Horus not only forgave it, when he learned of what had happened on Olympia he lauded his brother's thoroughness and dedication to eliminating traitors to the Imperium. Horus swore Perturabo never to feel guilt over what he had done to Olympia, but that was an oath easier to make than to live by.This welcome absolution combined with Perturabo's own guilt and bitterness towards his treatment by the Imperium to lead the Lord of Iron to side with Horus in his plans to replace the Emperor on the throne of Terra.As such, when the Iron Warriors were ordered to help put down Horus' rebellion on Istvaan V, the Iron Warriors instead turned on their fellow Loyalists and joined the Traitors' cause at the Drop Site Massacre.