Perturabo was leading his Iron Warriors on a campaign to cleanse the [[Hrud]] Warrens on [[Gugann]] when he recieved received news that his homeworldhome world, Olympia, was in rebellion. Dammekos was long dead but demagogues had brought the people together and risen in arms. The thought of being the only primarch who could not control his own homeworld home world appalled him. Horus made the most of the opportunity by presenting Perturabo with a hammer named [[Forgebreaker]]. It is possible that this weapon was one by which the powers of Chaos could influence Perturabo and manipulate his thoughts. Alternativly Alternatively it could have been seen to be a signing of a pact, a gift from Horus to Perturabo. Perturabo purged Olympia city by city, overruning overrunning the fortresses he had built and sparing no one. By the time the massacre was over, five million Olympians had been killed and the rest put in into slavery. Perturabo looked on in cold silence. After the pyres were burning, the Iron Warriors realised realized what they had done. They had changed from the saviors of the [[Imperium]], destroying the Hrud one moment and the next they were committing genocide. Perturabo realised realized the Emperor could never forgive him for what he had done.

It was in that time that news came through, Leman Russ and the [[Space Wolves]] had attacked [[Magnus]] and his [[Thousand Sons]] on [[Prospero]] and [[Horus]] had turned renegade with his [[Sons of Horus]] Legion. There Their orders were to meet with six other legions to face Horus on [[Istvaan V]]. The result of the conflict was that the Iron Warriors, Night Lords, Word Bearers and Alpha Legion all joined with Horus and destroyed the three remaining loyal legions.

After Istvaan V the Iron Warriors were let loose and Perturabo relished the opportunity to fight in a way that did not rely on massive sieges and trench warfare. Because of their massive deployment throughout the galaxy, dozens of [[Warsmith]]s took over planets and demaded demanded tithes to support the heresy. A large contingent of the Iron Warriors accompanied Perturabo to [[Terra]] where he supervised the bombardment and siege of the [[Emperors Palace]]. Perturabo took a perverse pleasure in tearing down the defences created by the vain (and often annoying) Rogal Dorn. After the invasion failed and Horus died, the Iron Warriors fled to the [[Eye of Terror]], like many of the other traitor legions. Here Perturabo secured a new planet named [[Medrengard]] and plotted the downfall of the Imperium.