Marius Gage - The Osiris Rebellion: By the third decade of the Great Crusade, the XIIIth Legion had begun to develop a distinct character of their own. By the year 833.M30, the XIIIth Legion had increased in number to an active force of around 33,000 Space Marines, whose primary battle group now consisted of the autonomous 12th Expeditionary Fleet. Such was the size, self-sufficiency and stability of the XIIIth Legion that the 12th Expeditionary Fleet was entrusted with the task of exploring the extent of the inner galactic disc and its densely packed star systems coreward of Terra. This was why when a secession crisis unexpectedly flared up close to the capitoline systems of the Segmentum Solar itself, the XIIIth Legion was the closest available Legion and the swiftest to respond.The Osiris Cluster, a grouping of eleven star systems making up the inner portion of the Segmentum Solar's second quadrant, had suddenly and without warning declared secession from the Imperium. Chartist merchant vessels had been seized, Imperialis Armada naval patrol squadrons fired upon and driven off by system defence ships, and agents of Imperial authority rendered silent and assumed dead. The inhabited worlds of the Osiris Cluster, many technologically advanced at contact, had originally come into Compliance during the Great Crusade's eighth year in relatively bloodless order. It had been viewed as a highly successful campaign in which the XIIIth themselves had a hand. That the Osiris Cluster had now fallen into open rebellion was deemed an affront to the Legion's honour by its Lord Commander, Gren Vosotho, and the master of the XIIIth had vowed to bring the matter to a resolution as swiftly and emphatically as possible. Vosotho, acting on initial intelligence reports, ordered the warships of the 12th Expeditionary Fleet to proceed directly to the halo world of Septus XII, leaving behind the fleet's support elements, lumbering transports and forge ships guarded by its slower combat vessels. The target of this rapid strike force was to be the atmospherically-sealed hive city of Cabasset, located on the night-side of Septus XII, and the economic and political capital of the Osiris Cluster. A strike there before the rebels had time to consolidate their forces, Vosotho reasoned, might end the rebellion in a single, bloody stroke.Close support fire from the XIIIth Legion's flagship, Sethaln's Thunder, opened up great rents in Hive Cabasset's protective outer shell into which Vosotho personally led his Legion's Stormbirds to the attack. Resistance was immediately far heavier than expected as the landing force became swiftly bogged down in human attack waves made up of at first hundreds and soon thousands of dead-eyed civilians crudely stitched into makeshift pressure suits and armed with improvised weapons of every sort, not least among them explosive mining charges converted to suicide devices. The XIIIth quickly modified their tactics to inflict maximum attrition but, heedless of casualties, the tide of bodies pressed on in cold silence and it was quickly apparent that this was no mere rebellion and no ordinary enemy. Unwilling to allow his attack to be stalled and his invasion force surrounded, Vosotho called down reinforcements and ordered his attacking squads to press on, relying on speed and co-ordination, as well as the superiority of his Space Marines in close quarters, to carry the battle. But unknown to Vosotho, the Legiones Astartes were being led into a cunningly laid trap. As the XIIIth Legion pressed their attack deep into the main hive city, a xenos fleet of unknown type and origin, comprising five vast hour-glass shaped vessels whose structure turned and rotated ceaselessly like clockwork, appeared with great speed from within the fiery corona of Septus' giant star. Realising the disaster that was about to unfold, Vosotho called a general retreat from the surface, but as his forces battled to return to their gunships and transports, the assault on the Space Marines intensified as the nature of the attacks began to change.While the waiting Stormbirds fell under concerted all-out attack in an attempt to cripple or destroy them, fresh mobs of grasping civilians poured from side-junctures and corridors, their intention not to kill but to overwhelm and pinion individual Legiones Astartes, drowning them in their mass of bodies, heedless of the cost in life. Above them in the void, the two fleets clashed. The great hourglass craft, each easily out-massing the gargantuan Sethaln's Thunder, lashed out with blazing whips of elemental particles, scorching and burning the Imperial warships and engulfing any fighter squadrons or torpedo salvos that came close in collapsing gravitational singularities, annihilating them utterly. The 12th Expeditionary Fleet was overmatched but fought on valiantly, causing one of the titanic xenos craft to fall back, strangely-coloured vapours bleeding luminously from its rent hull, but at the cost of a dozen of its own number, while the Sethaln's Thunder, at that point a burning wreck, tumbled out of control through the line of battle.It was then that fearful figures, aglow from within with sickly light, began to materialise among the attackers both on their surface and to teleport directly onto the warring Imperial vessels. Wearing baroque bio-mechanical containment suits, the ghoulish shapes of glowing mists used gauntlets that spat ethereal fire and their alien wills reached out to crush the minds of those who resisted them. Vosotho's final command was for the fleet to withdraw with as many of the XIIIth Legion that could be recovered, but withdraw it must. A new enemy of the Imperium had been met, and word of it must reach Terra at any cost. Vosotho committed his life to the command of the rearguard on the surface, taking penance for his error, and his last act was to transfer Legion command to the most senior surviving commander present in orbit, Marius Gage. It was a testament to Gage's swift thinking and tactical acumen that he was able to hold off the enemy vessels until every surviving Stormbird from the ground assault had departed the planet, fighting a swirling three-dimensional battle of thrust, counter-thrust and retreat which held the enemy titan-ships at bay until the 12th Expeditionary Fleet had fought its way clear.Ultimately, what could have been a disaster had been fought into a mere defeat, and when all was afterwards measured, the XIIIth Legion had suffered a little over 6,500 Space Marines lost, the largest tally of any single battle in the XIIIth's history. Although this was approximately a fifth of its fighting strength in terms of Legiones Astartes, the lost counted among them much of the elite of the Legion, many of them Terran veterans from its founding and its Lord Commander Gren Vosotho with them. Its fleet had also suffered severely, with a quarter of its warships lost or irrevocably damaged, not least of all its flagship. Just as bitter a blow was to the Legion's pride and honour at the defeat to which their overconfidence had led them. They hungered for vengeance but even this was denied them. When the XIIIth Legion returned to the now quarantined and blockaded Osiris Cluster a little under a standard year later with a force heavily augmented from the Solar Armada, elements of the XVIIIth Legion and specialist anti-psyker cadres from Terra, of the xenoforms the Officio Biologis had designated with the cognomen of "Osiran Psybrid", they instead found worlds either left as wastelands of the unburied dead or locked in turmoil and civil strife. But of the architects of these atrocities they found no sign.In the wake of the short campaign, empty of glory, which brought the remains of the Osiris Cluster back under the Imperium's control for repopulation, the XIIIth swore a blood oath for vengeance against the xenos wherever and whenever they might appear again. The Legion, now under Marius Gage's stewardship, reorganised and sought to quickly replenish its numbers and supplies, and afterwards redoubled its effort in the Great Crusade's service, as if trying with each fresh victory to prove that the defeat at Septus had been an aberration never to be repeated, and the name of its lost master and indeed the battle in which he fell became a thing no longer spoken of but which dwelled as a shadow at their shoulders.