Iron Warriors - The Horus Heresy: As the tragic outbreak of the Horus Heresy grew closer, it appears that Perturabo was put under ever-increasing pressure, and as a result the fires of his bitterness were stoked to a raging inferno. Some have postulated that it was the Warmaster Horus who, time after time, engineered events and adjusted deployments to the Primarch's psychological detriment.For the Iron Warriors Legion the Horus Heresy came as the culmination of a series of reversals and fell tragedies that had occurred in the latter years of the Great Crusade, stalked the Legion and by their effect both deranged and twisted is Legionaries.Foremost of these had been the rebellion of Olympia, the seat of the Legion's domain in the Meratara Cluster and foundling homeworld of their Primarch Perturabo. With the death of the long-lived Tyrant of Lochos and Satrap of Olympia, the duplicitous and viperous politics of Olympia had severely degraded into infighting and insurrection.The violence and division flared up worse than ever before because of the changes the Imperium had wrought onto Olympia after bringing it to Compliance, 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 Legion and its master, and could not have come at a worse time, because for over a Terran year, the Iron Warriors Legion had been engaged in the almost single-handed suppression of a major infiltration of the infamous xenoform known as the Hrud. All such actions in the history of the Great Crusade had proved costly both in terms of lives and the sanity of those who must fight such nightmares, and this was to prove the exception.In the midst of the cleansing of the Hrud Warrens on the world of Gugann, the IVth Legion was notified of the rebellion of its homeworld. It was Horus himself who broke the news to Perturabo that his homeworld of Olympia was in rebellion against the Imperium. Dammekos had died and the population had taken up arms against the Imperium following years of relentless anti-Imperial propaganda by the dead Tyrant.Perturabo was by this time tired of repeatedly having to prove his worth, and the thought of being the Primarch of the only Space Marine Legion unable to hold its own homeworld appalled him. Horus bade Perturabo to return to his place of discovery and presented him with the Power Hammer Forgebreaker, which is believed by some Imperial scholars to have acted as a conduit through which the Ruinous Powers could manipulate the Iron Warriors' Primarch.Perturabo's long-repressed anger was finally unleashed, and upon his return to his homeworld, the Primarch enacted such fearsome vengeance that countless innocents were slaughtered and entire cities burned. Perturabo and the Iron Warriors brutally suppressed the rebellion on the streets of the city-states of Olympia.No one was spared. It was the principle of surrender or no quarter, and the Iron Warriors had grown accustomed to granting no quarter. Perturabo watched as the Olympian fortifications in which he had once taken such pride were overcome. By the time the massacre was over, Olympia had been culled into slavery. Five million civilians had been killed in the process.As the pyres burned through the long Olympian night, the Iron Warriors slowly realised the extent of what they had done. One moment they were humanity's heroes assaulting the hideous alien Hrud and the next they were committing genocide against their own people. In the aftermath of his vengeance, Perturabo knew utter despair, barely able to comprehend the crimes he had committed in his rage. He knew that the Emperor could never forgive him for his deeds.In the wake of this tragedy, the IVth Legion retreated further into a private world of stark violence and bitter paranoia. Its brutality suppressed the worlds it garrisoned with renewed savagery and unleashed genocidal force upon the first hints of open rebellion, with many of its commanders growing ever more unhinged and isolated from the Imperium they served.But before Perturabo could set about righting his terrible deed, word came of Horus' virus-bombing of the Traitor Legions' remaining Loyalists at Istvaan III, and the Iron Warriors were ordered by the Emperor to confront the Traitors and bring them to justice.The Iron Warriors also received news of the most inconceivable kind: Astartes had slain Astartes. The news would have been shattering under normal circumstances, but when heard amidst the ruins of a world that were thick with the stench and corpses of the dead, it was apocalyptic.Not long after, news arrived that Leman Russ had led his Space Wolves in an attack upon Magnus the Red and his Thousand Sons upon their homeworld of Prospero for Magnus' continued violation of the Imperial Edicts of Nikaea forbidding the use of psykers by the Astartes and the practice of psychic sorcery.Horus had turned Traitor to the Emperor along with his own Sons of Horus Legion. Angron's World Eaters and Mortarion's Death Guard were also now Traitor Legions. Fulgrim and the Emperor's Children had also declared their allegiance to Horus' cause and Chaos, and the Renegade Primarch had unsuccessfully tried to turn Ferrus Manus of the Iron Hands to the worship of the Ruinous Powers. The Iron Warriors were ordered to join the Iron Hands and five other Loyalist Legions in a task force intended to crush the nascent rebellion against Imperial rule in the Istvaan System.Between their bitter rivalry with the Imperial Fists and the guilt derived from their butchery on Olympia, in retrospect it was no surprise that Perturabo and the IVth Legion turned Traitor to the Imperium. History records little of the machinations Horus must surely have enacted in order to turn the bitter Perturabo to the cause of the Traitors, but whatever the truth, the Iron Warriors turned upon their brothers at the Drop Site Massacre on Istvaan V, and in so doing sealed their damnation and allegiance to the Ruinous Powers.On Istvaan V, the Iron Warriors, the Night Lords, the Word Bearers and the Alpha Legion were held in reserve, while Ferrus Manus led his own Iron Hands, along with the Raven Guard and the Salamanders, against the Traitor positions in the first Loyalist wave.After being heavily engaged with Horus' forces, the surviving Loyalists of the first wave eagerly sought the shelter of the Iron Warriors' trenches and bunkers, only to be mercilessly gunned down by their erstwhile allies. Henceforth, the Iron Warriors have always been known as the "Betrayers of Istvaan" in the wider Imperium.