Treachery at Port Maw - A Muster for War: The Manachean Commonwealth was an independent dominion forming its own self-governing sub-sector within the Imperium to the galactic East of the Cyclops Cluster. Considered the lynchpin of the Coronid Deeps, it was also the principal axis of Imperial power holding the northern border of the Imperium together thanks to the dual influence of two star systems: Manachea itself - principally the Hive World of Manachea Vysidae - which was in terms of population and industry the equal of any of the great Hive Worlds of the Segmentum Solar, and Port Maw which, while still under construction and expansion, was one of the most powerful Imperialis Armada bases in the northern Imperium. This border sub-sector, geared as it was for war, presented a formidable obstacle to any attack contemplated against it, and few regions outside the heartlands of the Segmentum Solar could claim to be so well protected.Port Maw itself was home to a very sizable fleet, numbering some one hundred and seventy capital and line vessels of the first rank, and nearly a thousand other warships of lesser degree, whose principal purposes were to patrol the nearby Segmenta within the borders the Great Crusade had established; defend its worlds from attack, maintain order and enforce the writ of Imperial law. As part of this peacekeeping force, stationed there at secondary bastions in the Manachea and Numinal Systems, was a combined army of one hundred and ninety-three Solar Auxilia Cohorts of the Imperial Army, forming an independent strategic command of the Excertus Imperialis under Grand Admiral Ospheus LaBray.In practical terms, this comprised notionally some 3,860,000 fully armed, equipped and disciplined soldiers outfitted to the highest standards of the Imperium's human soldiery. As Solar Auxilia, they were trained and experienced both in void combat and planetary defence operations, and represented a significant military force, even when set against the threat of a Legiones Astartes attack. Besides this elite army, the Commonwealth's own emergency defence planning was predicated upon the ability to quickly raise, arm and supply tens of millions of planetary militia in extremis, and this act itself did not seriously impact upon the massive economic output of the sector -- such were the immense labour reserves of Manachea Vysidae and the war matériel its wealth had allowed it to stockpile.Nor was the Commonwealth entirely without the assistance of the Legiones Astartes, albeit in very small numbers. The limited garrison of Imperial Fists stationed in their bastion on Manachea Lux had quickly subdivided itself and sent its Legionaries to act as advisors throughout the Commonwealth, lending their expertise in counter-siege warfare to improve defensive structure and emergency action plans. However, those of the Astartes that had managed to escape the deadly trap set for them on Istvaan V refused to submit themselves to "merely" human commanders and had pursued their voyage, grateful for the opportunity to regroup and re-equip before moving on.The question then immediately becomes: why did this vast and mobile force not come to the aid of the Cyclops Cluster when it was attacked? The answer is a simple one; it was not allowed to. The Warmaster Horus had not allowed it.Long before the first shot was fired on Istvaan V, or even Istvaan III, the Warmaster had ensured that every centre of the Imperial military he could not control or vanquish immediately would be temporarily neutralised so as not to impede his first moves in the planned galactic civil war. So it was that at the time of the Istvaan III Atrocity, numerous elements of the Port Maw Excertus had been deployed to the very edges of their usual far-flung operational range; tied down in chasing half-unsubstantiated reports of privateers or xenos marauders, occupied on deep patrols or even temporarily reassigned to other campaigns or fleets in distant border regions of the Imperium -- whether such reinforcements were truly warranted or not. With the authority of the Warmaster still absolute and uncontested, such actions didn't raise any suspicion or comment. When the rebellion became apparent, orders were modified and warship squadrons and Cohorts of Solar Auxilia recalled at once for a grand muster at Port Maw, but this was a task neither swift nor easy. Marshalling troops also became increasingly more difficult as a great increase in Warp Storms and Warp turbulence was noted which would only gain in intensity as the Horus Heresy unfolded. By 006.M31 matters became ever more dire for the isolated Port Maw Excertus as its fleet elements had spent most of the previous years chasing shadows and were now confronted with a very real and terrible enemy. All across the Coronid Deeps, reports of isolated and unknown warships, civil unrest, outlaw activities, unexplained calamities and xenos raids became ever more frequent -- then came the messengers of the Warmaster, demanding the Dark Compliance of the Cyclops Cluster, the Grail Abyss and even Lethe within the borders of the Manachean Commonwealth. It was only a question of time until such an emissary would be sent to Manachea as well.Yet no emissary came, but that seemed a small relief compared to the dire news that continued to reach Manachea. Many deep range patrols sent beyond the Segmentum border simply did not return, while raiders attacked Imperial outposts perilously close to the borders of the Coronid Prohibition Zone, an area of space where the authorities of Port Maw were forbidden to enter by ancient decree. These raids were tentatively attributed to the Rogue Trader Militant Rom Jhutlannder whose ships had served in the past as outriders and pathfinders for the Warmaster's own 63rd Expeditionary Fleet. However, when the enemy finally manifested itself, it would not be from the quarter expected, but from the western approach of the Commonwealth: a xenos-infested conglomerate of ancient ship-wrecks and trapped asteroids, a Space Hulk which the Manachean Commonwealth had not seen in many decades -- the Orks had invaded the Vlorath Srader System.