Perturabo was leading his Iron Warriors on a campaign to cleanse the [[Hrud]] Warrens on [[Gugann]] when he received news that his home 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 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 and manipulate the thoughts of the wielder. 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, 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 into slavery. Perturabo looked on in cold silence. After the pyres were burning, the Iron Warriors realized what they had done. They had changed from were no more the saviors saviours of the [[Imperium]], destroying the Hrud one moment and the next they were committing genocide. Perturabo realized the Emperor could never forgive him for what he had done.

After that 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 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 [[Emperor's Palace]]. Perturabo took a perverse pleasure in tearing down the defences created set up by the vain (and often annoying) Rogal Dorn.

After fleeing Terra following Horus' death, Peturabo Perturabo took the opportunity to take vengeance on the Imperial Fists with a trap on Sebastus IV. The trap was known as the Eternal Fortress, a keep centred within twenty square miles of bunkers, towers, minefields, trenches, tank traps and redoubts. Upon hearing of this, Rogal Dorn publicly declared that he "would dig Perturabo out of his hole and bring him back to Terra in an iron cage".

Rogal Dorn expected an honourable battle, but this was not to be. Beginning by isolating the four Companies of the Imperial Fists from their orbital support, Perturabo began to carefully divide his enemy and destroy them piecemeal. Some Imperial Fists managed to penetrate the defences and reach the center centre of the Eternal Fortress, only to find there was no central keep - simply an open space watched by yet more defenses. The fortress was a decoy of no real value, surrounded by twenty miles of killing ground. By the sixth day of the siege, Imperial Fists Space Marines were fighting individually, without support, using the bodies of their own battle brothers for cover.

The siege of the Eternal Fortress (later referred to as the "Iron Cage") lasted for a further three weeks. Relief came in the form of Roboute Guilliman and the Ultramarines, but the siege left Rogal Dorn a broken man and rendered the Imperial Fists Chapter unable to fight for nineteen years. The gene-seed of over 400 Imperial Fists was sacrificed to the Dark Gods, and Petruabo Perturabo was elevated to the rank of [[Daemon Prince]] of [[Chaos Undivided]].