Auretian Technocracy War

Prelude to War
The Auretian Technocracy War was a conflict fought towards the end of the Great Crusade between the forces of the Imperium and that of the eponymous Auretian Technocracy, an advanced human civilization. After repeated attempts to bring the Auretians into the Imperium though diplomatic means failed, due in no small part to the extremely independent nature of the Auretian people. Thus the Emperor decreed that they would be humbled and tasked two of his sons, Lukas Keath of the Obsidian Guard and Hectarion Mycenor of the Crimson Lions, with conquering the Technocracy.

Course of Events
Initially the Imperium won a string of victories, sweeping aside the pitiful Auretian Navy and destroying the orbital defense stations in short order. When the time came to begin the planetary assault things became more complicated, as the Emperor had also dictated that as much of Aureus's industrial capacity and technological advancements could be preserved would be, ruling out an orbital bombardment and meaning the planet had to be taken the hard way. So the Legions launched simultaneous attacks via drop pod on key locations across the planet such as the surface-to-orbit laser and missile batteries, power plants, supply depots, and manufactorae. Although caught off guard the Brotherhood forces, who wore power armor and wielded bolters, put up stiff resistance to the invaders. In many places progress ground to a halt while in some others it was even reversed as the Brotherhood counterattacked and forced the Legiones Astartes back. Despite the valiant effort on the part of the Brotherhood they simply did not have the numbers to properly defend everywhere for long, so gradually each stronghold fell, its defenders slaughtered to a man.

With a stable beachhead the Imperium were able to bring in fresh supplies and reinforcements, chief among them the Titans of Legio-Praesagius. Supported by the firepower of over two dozen engines the Obsidian Guard and Crimson Lions made considerable progress, securing many critical locations. However the Brotherhood remained a dangerous enemy as proven by the ensuing counteroffensive. The repeated defeats turned out to be a blessing in disguise for the Brotherhood, as their lines of supply and reinforcement became much shorter while the Imperial troops were dispersed to cover more territory. A glaring weakness that the Brotherhood exploited to devastating effect. In the span of a week the progress of the previous month was undone, only Legio Praesagius's presence prevented a total defeat. Slowly but surely the Imperium regained the lost ground and, learning from their mistake before, made sure to fortify each district before moving on.

Finally, after half a year of hard fighting, the Brotherhood was pushed back to the Iron Citadel, the greatest fortress on the planet. When confronted by such an obstacle Lukas and his Obsidian Guard scoffed then proceeded with a month long bombardment that reduced the defenses of the Iron Citadel to rubble and slag. Legio Praesagius spearheaded the assault with the Primarchs and their legions following just behind. The storming of the inner keep was a short and bloody affair, cornered and with no hope of survival the remaining Brotherhood soldiers fought ferociously to kill as many Astartes as possible before dying. Eventually Lukas and Hectarion entered the Fabricator Consul's chambers, and as the former held Salignac down the latter severed his head in a single blow.

Aftermath
As major combat operations came to a close the bulk of the Legiones Astartes prepared to live with only small contingents from both legions staying behind to assist in the pacification and reconstruction. Also located within the Iron Citadel was the STC which had provided the basis upon which all Auretian Technology was based. Together the brothers presented the head of Salignac, preserved in a stasis field, and the STC to the Emperor as confirmation of their victory.