Dark Angels - Battle of Diamat: El'Jonson's forces were too deeply enmeshed in the Shield Worlds of the Gordian League to respond quickly to Horus' betrayal; the best estimates of the primarch's staff indicated that it would take them nearly eight solar months to conclude their offensive operations, even on an emergency basis, and reposition themselves for a strike against Isstvan V.Even if they could move more quickly, Horus' agents would be able to alert the Warmaster in time to organise a counter-strike. However, Jonson believed that a small, hand-picked force might accomplish what an entire Legion could not.The primarch issued orders for many of the Dark Angels' 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 Isstvan V against the approaching Loyalist strike force.El'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. El'Jonson went to Diamat in order to secure several powerful continental siege machines known as Ordinatii; 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 star port. With no way to secure the siege machines held in storage in Diamat's depots from El'Jonson's Astartes, the admiral of the raiding fleet had little choice but to withdraw back to the Isstvan System. The Warmaster's final gambit had failed.Following this small victory at the Battle of Diamat, El'Jonson met with his fellow brother primarch, Perturabo of the Iron Warriors Legion, aboard his flagship Invincible Reason. Perturabo informed Jonson that the IVth Legion was en route to the Isstvan System, to face the Warmaster and his Traitor Legions upon the black sands of Isstvan V.Ferrus Manus and the Iron Hands Legion had hastened ahead of them, hungry to claim the Emperor's vengeance against Horus. Perturabo lied to El'Jonson, explaining that he had hoped that his Legion could provision his vessels at the Xanthus star port above Diamat before continuing to the combat zone. Of course, they were now unable to, as the Ist Legion had destroyed the orbital port.Perturabo inquired of El'Jonson how he had learned of the existence of the Ordinatii siege engines. Jonson explained that he had discovered them fifty Terran 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.Horus had commissioned the colossal siege machines from the 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 on Diamat against the day when the Warmaster would come to claim them. But then the Isstvan III Atrocity occurred. When Jonson had 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 Isstvan System, the Warmaster couldn't claim total victory so long as the Emperor was safe in His palace. For Horus to triumph, the Emperor 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.El'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 of the Gordian League and prepare the rest of the Ist 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. El'Jonson explained that his actions were done for the good of the Imperium, not for accolades, nor for power.But El'Jonson confessed to his brother a certain jealousy. He believed Horus had become the Emperor's favourite son for no other reason than fate. Had he been the first of the primarchs to be found, El'Jonson believed he would have been the Warmaster instead.The Lion also 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 ultimately reached an understanding -- El'Jonson granted permission for the Iron Warriors to take possession of the siege engines at their convenience -- on one condition. El'Jonson made his brother promise that the Ordinatii would be put to good use.Perturabo assured his brother that they would be, never letting on that he had already sworn himself to Horus' rebellion and would participate only solar months later at the Drop Site Massacre on Isstvan V where three Loyalist Legions were almost completely destroyed.