From beyond the Sea of Haunted Souls death came
Since the combined forces of the Tau Empire’s Cadre of the Killing Blow and Chaos Space Marines warband the Lords of Ascension destroyed the Imperial Space Fortress controlling the passage through the Sea of Haunted Souls Nebula; war had been expected. The Xenos and Traitor armies had destroyed the bulk of the Imperial forces along with the fortress, leaving the sub-sector without proper support from the Astra Militarum and Imperial Navy. The local planetary defence forces dug in, praying for reinforcements and the Emperor’s protection. For most of them, their prayers would go unanswered.
While the Chaos Space Marines descended into slaughter, murdering their way across a dozen worlds, Tau expansionist forces pushed past the Nebula into Imperial space. Several new colonies were established, some even without having to resort hostilities. Both armies made great progress, rapidly advancing towards the true price: the Forge World of Mons Falco. The surviving Imperial commanders knew this and ordered the remaining Cadian regiments fall back to the planet. Mons Falco must stand lest the entire sub-sector fall.
On the northern brim of the Sea of Haunted Souls Nebula lies a string of star systems known as the Promethium Shores. The star systems each hold many gas giants and promethium mining facilities. At the heart of the Promethium Shores lies the mighty Forge World Mons Falco. The planet is truly the crown world of the sub-sector. It’s regional power comes not from its proximity to valuable resources nor from its own wealth of labour alone.
Warp travel is never to be taken lightly but on the Eastern Fringe the dangers are far greater. The Eastern Fringe is notoriously difficult to navigate, as the guiding light of the Astronomicon grows dim by distance. Without the unchanging astrocartographical reference point of the Astronomicon the Navigators plot the courses all but blindly. Herein lies the true value of Mons Falco to the Imperium of Man. Upon the surface of Mons Falco resides a vast archeotech construct known as the Eastern Gate.
The origins and purpose of the Eastern Gate are unknown. The tech-priests have yet come to comprehend the enigmatic archeotech construct. But the following is known; the Eastern Gate
create a stable and protected warp anomaly. To the Navigators it functions as a beacon of light and thus enabling more stable and swifter warp travel to the sub-sector.
Recently the tech-priest have found a way to increase the energy levels of the Eastern Gate. The warp light shines ever brighter and warp travel have become even safer. Reinforcements have reach the depleted Imperial forces faster than expected enabling them to fight on. But no reward comes without risks; the tech-priests’ manipulation the archeotech device have caused the northern glaciers to melt leading to flooding of great areas and the loss productive capability. The Imperial Command deems the side effects as unfortunate but acceptable.
The Astra Militarum on Mons Falco tracked the movements of both the Chaos Space Marines and the Tau vanguard forces with extreme scrutiny. Biding their time they dug in deep around the strategic areas preparing for the unavoidable confrontation. Needless to say they were taken by complete surprise when a mighty Saim Hann warhost struck. From webway portals previously buried deep beneath the frozen northern wastelands the Eldar launched a ferocious attack wresting control of the Eastern Gate from the 103rd Cadian regiment.
The battle was hard fought. Even if the Eldar had blindsided Cadians completely the battle raged for weeks but in the end the 103rd broke and abandoned the Eastern Gate to Saim Hann raiders. As the Astra Militarum launched a counter attack in order to reclaim lost control of the vital archeotech the Tau forces made planetfall less than a month later the Chaos Space Marines too had arrived. Total war had erupted on Mons Falco.