With a hammer gifted from Horus himself, Perturabo purged his own homeworld, Olympia, destroying fortresses and slaughtering everyone.
Via our good friends at Lexicanum
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, the gift symbolic of a signing of a pact between them.
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 were no more the saviors 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. Horus, on the other hand, commended Perturabo for his decisive action and the Lord of Iron swore an oath of loyalty to the Warmaster.
It was in that time that disturbing news of the Horus Heresy and new orders came from Terra. Leman Russ and the Space Wolves had attacked Magnus and his Thousand Sons on Prospero. Horus had turned renegade with his legion the Sons of Horus besides some other legions. The whole of the Imperium was on the brink of a civil war.
The new orders were to join with six other legions and then to face Horus and his forces on Isstvan V. The result of the following battle was that the Iron Warriors, Night Lords, Word Bearers and Alpha Legion all joined Horus and almost completely destroyed the three remaining loyal legions on the Drop Site Massacre. 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 Warsmiths took over planets and demanded tithes to support the heresy.
After the First Siege of Hydra Cordatus Perturabo was approached by Fulgrim, Primarch of the Emperor’s Children, who promised the Lord of Iron near unlimited power. The two embarked on a quest to tip the balance of the rebellion in Horus’ favor by journeying into the Eye of Terror and retrieve a weapon known as the Angel Exterminatus. After a brief boarding action with the Sisypheum (which Perturabo allowed to escape due to his disgust of Fulgrim’s Legion), the duo arrived in the Eye and quickly landed on the Crone World of Iydris.
Entering an ancient Eldar citadel known as the Amon ny’shak Kaelis, the Emperor’s Children and Iron Warriors had to contend with an army of Wraithguard and Wraithlords that had awoken from their slumber. At the height of the vicious battle, Fulgrim attempted to betray his brother and slip away but was pursued by Perturabo. Both soon arrived in a massive spherical chamber, and Fulgrim revealed to his brother that there was no Angel Exterminatus, but he rather was destined to become such a weapon on this world.
Furious, Perturabo attempted to attack Fulgrim but was drained of his power when Fulgrim activated the Maugetar stone that Perturabo had previously received under the guise of a gift. Now with the Iron Warriors Primarchs’ power in hand, Fulgrim revealed the purpose of his plan: to achieve Apotheosis, better known as Daemonhood.
However just as Fulgrim was about to complete his ritual and achieve Daemonhood, Perturabo gathered enough strength to charge at his brother but were interrupted by an ambush of Salamanders, Iron Hands, and Raven Guard who had been stalking the traitor fleet in search of vengeance since the Drop Site Massacre. One of the loyalist Astartes during the battle shattered the Maugetar stone, freeing some of Perturabo’s energy and giving him the ability to strike down Fulgrim with his Thunder Hammer.
However, Perturabo only managed to destroy the Primarchs mortal skin and was reborn as a Daemon Prince. Now a massive but elegant serpentine creature, Fulgrim told his brother that they would meet again then vanished in a burst of Warp energy along with his Emperor’s Children.
Exhausted and betrayed by his brother he knew had now lost any trace of honor, Perturabo allowed the loyalist Astartes to withdraw and led his own forces off the Crone World. Looking to settle the score with Fulgrim and better understand the cosmic power he had witnessed, Perturabo ordered his fleet sail deep into the Eye of Terror.
Perturabo and the Iron Warriors next found themselves trapped by the singularity in the heart of the Eye of Terror. Perturabo gambles in his attempt to escape, diving straight into the Black Hole at its heart. Perhaps by sheer blind luck, they were transported far across the warp to the Tallarn System. Once arriving at Tallarn, Perturabo was made aware of the Black Oculus hidden beneath the planet, and he immediately drew up plans to invade. Eventually, the Iron Warriors search for the Black Oculus was exposed, and they are forced to withdraw from the planet.
Later, a large contingent of the Iron Warriors accompanied Perturabo to the Siege of Terra where he supervised the bombardment and siege of the Emperor’s Palace. Perturabo took a perverse pleasure in tearing down the defenses set up by Rogal Dorn.
