Horus Heresy - Fall of the Iron Warriors: As the tragic outbreak of the Horus Heresy had grown closer, it appeared that Perturabo, Primarch of the Iron Warriors Legion, had become ever more bitter over the fact that his Legion had often been used for ginding siege duties that resulted in little glory or recognition. Events eventually came to a head when, leading his Iron Warriors on a campaign to cleanse the Hrud Warrens on the world of Gugann, the Primarch received news that his own homeworld, Olympia, had risen up in rebellion against the Imperium. The Olympian demagogues had brought the people together and risen in arms against the rule of the Emperor of Mankind. The thought of being the only Primarch who could not control his own Legion's homeworld appalled Perturabo.Perturabo and the IV Legion returned to their homeworld and brutally purged Olympia of its rebels city by city, overrunning the fortresses he had built and sparing no one who stood against him. 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 IV 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 could ever atone for a worldwide genocide. His father would never forgive him so grievous a sin, but Horus had not only forgiven it, he had lauded his brother’s thoroughness and dedication. Horus had sworn Perturabo never to feel guilt over what he had done to Olympia, but that was an oath easier to make than to live by.It was at this time that disturbing news of the outbreak of the Horus Heresy on the world of Isstvan III reached Olympia and new orders for the Iron Warriors came from Terra. Leman Russ and the Space Wolves had attacked Magnus the Red and his Thousand Sons Legion on their homeworld of Prospero at the direction of the Emperor and as a result of the deceit of Horus. Horus had turned Traitor with his XVI Legion, the Sons of Horus, alongside some other allied Legions such as the Death Guard, Emperor's Children and the World Eaters. The whole of the Imperium was on the brink of an outright civil war. The new orders from the Emperor ordered the Iron Warriors to join with six other Loyalist Space Marine Legions to face Horus and his Traitor Legions on the world of Isstvan V. During the battle that followed the Iron Warriors, Night Lords, Word Bearers and Alpha Legion all went over to the side of Horus and almost completely destroyed the three remaining Loyalist Legions of the Imperial assault force -- the Iron Hands, the Salamanders and the Raven Guard -- at the infamous Drop Site Massacre on that world. In truth, Perturabo and the other Primarchs who turned Traitor on Isstvan V had already been seduced by the Warmaster and made their decision to stand with him against the Emperor, guaranteeing that the Traitors would field nine Space Marine Legions against the Loyalists.