Battle of Iydris
|Perturabo and Fulgrim battle on Iydris|
|Outcome||Fulgrim ascends to Daemonhood|
|Traitor Legions||Eldar||Shattered Legions|
| Primarch Perturabo
Lord Commander Eidolon
Warsmith Barban Falk
Captain Julius Kaesoron
Captain Marius Vairosean(KIA)
Chief Apothecary Fabius
|Unknown, if any||Captain Cadmus Tyro|
| 2 Space Marine Legions
Traitor Imperial Army
|Thousands of Wraith Constructs||Small force of Iron Hands, Salamanders, and Raven Guard|
|Heavy||Annihilated||Light to moderate|
After the Drop Site Massacre Fulgrim, Primarch of the Emperor's Children joined forces with Perturabo, Primarch of the Iron Warriors on the world of Hydra Cordatus. Promising the Lord of Iron perfection, 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. Shortly before they were to leave, Fulgrim was hit by a direct headshot from the vengeful Sisypheum crew member Nykona Sharrowkyn. Fulgrim survived the wound however thanks to the efforts of Fabius Bile. After a brief boarding battle with the Sisypheum that saw the ship escape yet again, the two Primarchs 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. The Wraith constructs clashed with the carefully planned defensive lines of the Iron Warriors aided by the Titans of the Legio Mortis and the debauched hordes of the Emperor's Children, neither side attempting to aid the other.
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 of Slaanesh. 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.