Osirian Psybrids - Attack on Septus XII: The attack began well enough, with the close support fire of the Sethaln's Thunder opening 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 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 XIII Legion 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 coordination, as well as the superiority of his Space Marines in close quarters combat, to carry the battle.Slowly and with steadily growing losses, the Legiones Astartes forced their way deeper into the hive city and one by one began to claim their tactical objectives, crushing better-armed but equally vacant-minded opposition formed of what had once clearly been the bodyguard cadres of the hive city's nobility. But once the XIII Legion was heavily committed and kilometres deep into the hive the trap was sprung.A xenos fleet of unknown type and origin, comprising five vast hourglass-shaped vessels whose structures turned and rotated ceaselessly like clockworks, 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 of 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 the surface and directly onto the warring Imperial vessels.Armoured in some form of baroque bio-mechanical containment suits, the creatures within were barely corporal; ghoulish shapes of glowing mists whose gauntlets spat ethereal fire and whose 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 XIII Legion as 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, as penance for his error, and his last act was to transfer Legion command to the most senior surviving commander present in orbit, First Master 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 XIII Legion had suffered a little over 6,500 Space Marines lost, the largest tally of any single battle in the Legion's history. Although this was approximately a fifth of its fighting strength in terms of 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 heavily, 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 XIII Legion returned to the now-quarantined and blockaded Osirin Cluster a little under a standard year later , it was with a force heavily augmented from the Solar Armada, elements of the XVIII Legion (Salamanders) and specialist anti-psyker cadres from Terra. Of the enemy xenoforms the Officio Biologis had designated "Osiran Psybrid," there was no sign. Instead, the Imperials found worlds either left as wastelands of the unburied dead or locked in turmoil and civil strife.Piecing together fragmentary records from planetary dataspheres and Human minds alike all but purged clean, it was impossible to know from where the xenos had come or where they had gone, only that they had operated covertly at first, insidiously claiming worlds outright; burning out the wills of their populations, stealing away some bodily into the void, and leaving the others to simply mindlessly perish by starvation or inaction in their absence.Other worlds they had sent first into rebellion and then strife through covert psychic domination of their rulers and manipulation of their population's fears. The revelation was of a foe perhaps not numerous, but both insidious and frighteningly powerful, a clear threat now marked for extermination by the Emperor's own writ.In the wake of the short campaign, empty of glory, which brought the remains of the Osiris Cluster back into the Imperium's control for repopulation, the XIII Legion swore a blood oath of vengeance against the xenos wherever and whenever they might appear again. The Legion, now under Marius Gage's stewardship as the new Lord Commander and Legion Master, 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 XII 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.There existed within the Legion now a brooding sense of loss and a canker of doubt in its own abilities, and in the hearts of its Legionaries grew the desire, always present but now lent keen impetus, to reunite with their primarch as a balm to all their ills, to become -- as they saw it -- whole. The dark irony was that more than two Terran years previously, unknown and withheld from the Legion's knowledge and before the events at Septus XII, their primarch had been located by the Emperor, but owing to the vagaries of the Warp, contact would not prove possible for several more standard years.During this period, brief as it was and yet seeming an eternity to the sons of the XIII, the Legion fought on with a relentless but joyless hunger for battle, taking world after world for the Great Crusade in rapid succession, but shunning now both the laurels of victory it once courted and the respect of its peers it once craved, until the hour of its salvation came at last.