Unfortunately, the early history of this Forge World has been lost down the long millennia. Diamat is an isolated Forge World located in that Tanagra System in the Segmentum Ultima and only 52.7 light years from the Istvaan System. It also happened to be one of the most heavily industrialised systems in the entire sector, with the Class I-Ultra Forge World of Diamat and more than two dozen mining outposts and refineries scattered throughout the system.
Historically, Tanagra was rediscovered by Horus' XVI Legion and became Compliant relatively early in the Great Crusade. It was a key logistical centre for the region ever since. For much of the Great Crusade, this Forge World helped manufacture mighty Ordinatus war machines for the Expeditionary Fleets of the burgeoning Imperium of Man.
The Horus Heresy
When word began to filter out of the Warmaster Horus's perfidy at Istvaan III through the various Loyalist Expeditionary Fleets across the galaxy, many of the various Legions of the Legiones Astartes were still busy prosecuting their campaigns of Imperial Compliance. One of these fleets were led by the Dark Angels Legion of Primarch Lion El'Jonson.
Jonson's forces were too deeply enmeshed in the Shield Worlds to respond quickly to Horus' treachery. However, Jonson believed that a small, hand-picked force might accomplish what an entire Legion could not. The Primarch issued orders for many of his reserve squadrons to resupply and prepare for immediate deployment to the Tanagra System. Their primary target was to secure the Forge World of Diamat. They could not afford to let the Warmaster acquire the substantial supplies and ordnance needed to fortify the world of Istvaan V against the approaching Loyalist strike force.
Jonson would personally lead the expedition to Diamat, with a battle group of fifteen warships. Secrecy was vital, as the Primarch was aware that the Warmaster's agents were more than likely tracking the Dark Angels' movements. Jonson went to Diamat in order to secure several powerful continental siege machines; vast artillery pieces that could devastate the most powerful fortifications.
The small fleet of Dark Angels vessels arrived in the Tanagra System just five solar days after the destruction of Horus' landing force at the Xanthus starport. Strangely, Diamat's god-engines of the Legio Gladius were mysteriously absent -- deployed in support of the 27th Expeditionary Fleet under the command of the Raven Guard Legion -- far to the galactic south. Most likely, this Legion had been deployed from its homeworld by the Warmaster so that the Traitor Legions could more easily conquer Diamat and secure military materiel required to reinforce their position on Istvaan V before the Drop Site Massacre.
Diamat was left with eight regiments of Tanagran Dragoons Imperialis Auxilia regiments, two armoured regiments and several battalions of heavy artillery to defend it, as well as an unknown number of Mechanicum troops and various other assets -- an all-together impressive array of force.
With no way to secure the siege machines held in storage in Diamat's depots from Jonson's Astartes, the admiral of the raiding fleet had little choice but to withdraw back to Istvaan. The Warmaster's final gambit had failed. Following this small victory, Jonson met with his brother-Primarch, Perturabo of the Iron Warriors Legion aboard his flagship Invincible Reason.
Perturabo informed Jonson that the IV Legion was en route to the Istvaan System, to face the Warmaster and his Traitor Legions upon the black sands of Istvaan V. Ferrus Manus and the Iron Hands Legion had hastened ahead of them, hungry to claim the Emperor's vengeance against Horus. Perturabo then lied to Jonson, explaining that he had hoped that his Legion could provision his vessels at the Xanthus starport above Diamat before continuing to the combat zone. Of course, they were now unable to, as the I Legion had destroyed the facility.
Perturabo then inquired of Jonson how he had learned of the existence of the siege engines. Jonson explained that he had learned of them fifty standard years earlier when he was studying the history of the Great Crusade and saw a reference to them in a despatch from Horus that had been sent to the Emperor. He'd commissioned the colossal siege machines from the tech-masters of Diamat during the long siege of the xenos fortress-states on Tethonus.
The war machines took much longer for the forge masters to complete than planned. By the time they were finished, the campaign on Tethonus had been over for a standard year and a half, and Horus had moved on to other conquests. So the weapons were put into a depot there against the day when he would come to claim them. But then the Istvaan III Atrocity occurred. When Jonson received word of Horus' perfidy, he knew that ultimately the Warmaster's path would lead to Terra. Even if he were somehow to prevail against Perturabo and the other Legions sent to confront him in the Istvaan System, the Warmaster couldn't claim total victory so long as the Emperor was safe in His palace. No, for Horus to triumph, their father had to die. And that meant a long and costly siege of Terra. Therefore, the Warmaster would come to claim the siege engines of Diamat.
Jonson informed his brother that he would be unable to accompany the Loyalist fleet to Diamat, as he had to make all haste to the Shield Worlds and prepare the rest of the I Legion for the trip to Terra. In fact, he thought it best if no one outside Perturabo, himself and the other Primarchs ever knew that the Lion was there. He didn't want the Emperor to believe he did anything at Diamat with an ulterior motive in mind. Perturabo agreed that it was both a prudent choice, and a very humble one.
Jonson explained that his actions were done for the good of the Imperium, not for accolades, nor for power. Jonson confessed to his brother that he believed Horus had become their father's favourite son for no other reason than fate. Had he been the first one the Emperor had rediscovered in the wilds of the galaxy, Jonson believed he would have been chosen as the Warmaster of the Imperium. The Lion believed that Horus would inevitably be defeated and that the Emperor would need to choose a new Warmaster very quickly if the Great Crusade was to continue. He asked for Perturabo's support.
The two Primarchs reached an understanding -- Jonson granted permission for the Iron Warriors to take possession of the siege guns at their convenience -- on one condition, of course. At that Perturabo raised an eyebrow. Jonson made his brother promise that the siege weapons would be put to good use. Of that, Perturabo ironically assured his brother, they most certainly would be.
With the two Diamat Ordinatii secured, the Iron Warriors also acquired corrupted mind-impulse technology which allowed their Legionary gunners to interface with their weapons so closely that they could target heavy ordnance with preternatural accuracy through the use of the Ironfire siege protocol. This allowed the Iron Warriors to unleash a rolling bombardment ahead of assault units closing on enemy positions. The use of the Ironfire Protocol later facilitated the rapid capture of Loyalist strongholds without the need to commit an entire Grand Company, thus serving the Warmaster's orders to the Iron Warriors to topple Loyalist fortresses ahead of the march on Terra.
- The Horus Heresy - Book Six: Retribution (Forge World Series) by Alan Bligh, pg. 194
- Fallen Angels (Novel) by Mike Lee