Battlefleet Scarus - 13th Black Crusade: In 999.M41, war came without warning to the Scarus Sector, and the opening moves by the forces of Chaos saw large number of plague-ridden space hulks entering Imperial space. Though many were intercepted by the Space Marines and destroyed, some managed to escape the Astartes' attention.Some of these foul vessels of contagion found their way into the Belis Corona Sector, spreading their disease and pestilence amongst the masses throughout the sector.Along with the adjacent Agripinaa Sector, they were some of the first to bear witness to the rise of hordes of infectious, undead zombies that threatened billions of people on many of the Imperial worlds within the sector. This plague was later named the Curse of Unbelief.While the defence of the Cadian Gate from the forces of Chaos continued to go badly upon the surface of many worlds in the Cadian System, in particular Saint Josmane's Hope and Solar Mariatus, the war in space was fought more on the Imperium's terms.The Cadian System had been the focus of Abaddon's primary fleet actions, and the Imperium's fortunes there changed from day to day. The situation was highly fluid and the Chaotic naval forces found themselves stretched to exploit the victories they had won.A fleet the size of which has not been seen since the end of the Gothic War eight standard centuries earlier had been dispatched from Cypra Mundi, and arrived at the world of Belis Corona in preparation for a massive Imperial counter-offensive into the Cadian Gate.The arrival of this vast armada allowed those Imperial vessels that had been fighting continuously since the beginning of the invasion a brief respite. Severely depleted ship’s companies were bolstered through indiscriminate press-ganging, and hasty repairs and refits were undertaken on those vessels most in need of them.The Imperial reinforcements were split into battlegroups, each tasked with bolstering the defences in a specific sector of the Segmentum Obscurus. The regions around the Eye of Terror encompass many millions of cubic light years, and only by the concentrated application of resources in those areas in most desperate need could the Imperial Navy hope to make inroads and slow, stall, and eventually repel the Chaos warfleets plaguing the region.A small number of Imperial Navy ships won a decisive victory when they managed to hold back a larger Chaos fleet at the Ilthirium Belt, buying time for Battlefleet Agripinaa to arrive and allow the remaining ships to retreat. This course of action however, left the Agripinaa Sector open to invasion and soon the worlds of this sector and the Belis Corona Sub-Sector was transformed into nightmarish realms of plague and pestilence.With Warp Storms rendering astropathic communications erratic, the Segmentum Obscuras High Command continued to experience enormous difficulties coordinating command and control of its forces. Communications from the more distant sectors was particularly affected, and Imperial commanders were advised that those communications they did receive from distant sources were not to be trusted, due to psychodynamic distortion.A particularly implausible report from the Scarus Sector indicated that in addition to Chaos attacks, the defenders there faced the menace of the Orks.After action reports cited Mordax Prime as the initial focus of a massive Ork incursion which occurred at the same time as the Chaos assault was first hitting the segmentum, and if this were just an isolated case High Command would normally have written off the communication as misinformation caused by inexperienced troops facing an enemy they had no experience with.However, every major star system in the Scarus Sector soon logged similar reports, so they had to be taken seriously until more detailed information was forthcoming.Abaddon had recruited massed warbands of Orks to act as mercenary auxilia to his Chaos Space Marine Legions for his 13th Black Crusade. Following up on terror raids and cunning feints by the Night Lords and Alpha Legion Traitor Legions many of the Orks banded together into ever-growing tribes that declared themselves as part of the "Green Krusade" (or Kroosade in some sources -- Ork spelling is always a matter of conjecture) in imitation of the grand assault of Chaos.The unstoppable force of the Green Kroosade inundated the Scarus Sector (Skar-Uz to the Orks). The Orks continued putting intolerable pressure on a number of worlds in the Scarus Sector, notably Lethe II and Mordax Prime.The Magi Biologis of the Adeptus Mechanicus believed these foul creatures to be working alongside Abaddon the Despoiler to further their own ends, though they did not consider it to be a true alliance as such. More likely the Orks were taking advantage of the opportunity provided by the Despoiler's invasion to launch new attacks against distracted Imperial targets.The situation on these worlds was grim indeed. In particular, Mordax Prime could not be allowed to fall into greenskin hands, for it was a Forge World of prodigious output and its loss would damage the Imperium's war effort. More to the point, the Imperial forces dreaded the monstrosities the Orks might turn the production lines of Mordax Prime to creating.Fifteen Astra Militarum army groups and 5 legions of cybernetic Mechanicus Skitarii troops continued to hold Mordax Prime, yet clearly such a large body of troops was insufficient to stem the green tide engulfing that Forge World.The Segmentum Obscurus High Command drew up plans to send a further 35 to 40 regiments of the Astra Militarum to the world, drawn from the regional reserve to help bolster Mordax Prime's defences. Yet the Orks had set their sights upon capturing Mordax Prime, broadcasting their designs for the planet they referred to as "MoreDakka Prime" on every Imperial channel.Were it not for the fact that Mordax Prime was the sovereign preserve of the Adeptus Mechanicus, Imperial High Command seriously considered petitioning the Inquisition for an Exterminatus against the world before its vast resources fell to the enemy.