Osirian Psybrids - Battle of the Eurydice Terminal: Since his takeover of the Legion in the fourth solar decade of the Great Crusade after his rediscovery by the Emperor in 837.M30, Roboute Guilliman had succeeded in transforming the XIII Legion into an intricate and highly functioning weapon of war, and in doing so had built upon a track record of success in battle which held very few stains of defeat.For more than half a standard century the Legion had gone from strength to strength, waging Compliance actions and liberating Human-settled worlds for the Imperium across the eastern reaches of the galaxy, and forging the Realm of Ultramar in the process.Though not yet having reached the decisive numerical superiority of the other Legions, it would manifest by the time of the Horus Heresy, yet the Ultramarines of 899.M30 were perhaps already on the cusp of becoming the largest Legion, as their ranks, then standing at around approximately 166,000 Legionaries, stood them in the forefront of their peers.The Dark Angels, who in the previous decade to this had been undoubtedly the most powerful single Legion, had fallen in number and evened this figure, having suffered massive casualties holding the line during the famed Third Rangdan Xenocide. 50,000 Space Marines had spent their blood in preventing the destruction of perhaps the entire northern Imperium by the alien menace from the outer darkness.The Ultramarines in contrast had spent solar decades in building up their forces and expanding the frontiers in the galaxy's east, having had the advantage of their excellent network of supply and recruitment, and their primarch's formidable generalship was at a zenith of its strength thus far. Yet for all this, Roboute Guilliman knew that a shadow of doubt afflicted his Legion's soul. That doubt had its origins in its darkly storied defeat in the Osiris Cluster Rebellion a few short Terran years before the primarch's reunion with his Legion.Marius Gage's tenure in command as Legion Master had commenced with this disaster, and it became a baleful influence in the psyche of the Legion, a thing which while left unspoken nevertheless had power, and that even seemed to cast a pall over the Legion's recruits unborn when the battle was lost. The primarch knew that the only way to purge his Legion of this shadow of the mind was to find once more the xenos known as the Osirian Psybrids, and with the Ultramarines at his back, destroy them utterly.To this end, ever since he had first reviewed reports of the action when he took over his Legion, Roboute Guilliman had begun planning the Psybrids' destruction, analysing and re-analysing every facet of the Legion's battle-logs, gun-picter footage and even the deep auspex data gathered from the aetheric cogitators of the warships that had survived for microscopic fluctuations.In the solar decades as his Legion had gone from Compliance to Compliance, war zone to war zone, the primarch had never stopped running continued theoretical battle scenarios against the macabre and powerful xenos, knowing that one day they would make their reappearance and that he and his Legion would be waiting. But solar decades passed and they did not return.There had been rumours of course, unsubstantiated accounts passed on from frontier Rogue Traders, intermittent reports of inexplicable massacres and mass disappearances, but the galaxy, even within the fold of the growing Imperium, was a strange and dangerous place, and the evidence was seldom conclusive as to the culprit, or pointed to the more commonplace but no less deadly privations of several known xenos species.Only at Maxilla Veritas near the Maelstrom, twenty-six standard years after the Osiris Rebellion, was the evidence for the Psybrids involvement considered viable. But by the time fast cruisers from the Ultramarines fleet arrived, the trail was already long cold and the planet's death bore silent witness to the Psybrids' passing. It was in the closing segments of 899.M30 that conclusive word finally reached the Ultramarines primarch that the Psybrids had not only been encountered, but met in battle.Relayed from the central astrotelepathic chamber of Terra, the strange report had come from a sub-fleet of the XII Legion, then in its last days as the War Hounds before its own primarch, Angron, was found. The fleet, under the command of Praetor Erad Krüg, was fighting on the southwestern extreme of the Great Crusade's frontier near Eurydice Terminal. Here the War Hounds had been engaged in repelling an attack on the frontier outpost world by Ork void raiders from the self-styled Glortian Empire from the untracked abysses beyond.The War Hounds, though severely outnumbered, had held off the repeated Ork attacks through a series of savage voidship boarding actions in close orbit and high intensity assaults on their landing zones, preventing the xenos from gaining a foothold on the ground. It had been during the latest of these assaults, the largest yet attempted by the Orks, that a mysterious third party had attacked both sides.When the Ultramarines retribution fleet arrived, not even the superhuman intellect of the Ultramarines primarch could have predicted what his Legion would find there. The entire system had been sundered into a battleground littered with burning debris and the radioactive echo of heavy weapons fire. The Ultramarines found the last surviving few hundred Legionaries of the XII Legion, the remnants of a force that had once been ten times its number, led by the terribly wounded but still commanding Erad Krüg.The War Hounds praetor detailed to Guilliman the story of a strange and terrible battle in which the Psybrids had managed through their mental powers to enslave a vast feral population of Orks to do battle for them against their own kind, and had sought to do the same to the Imperial defenders of Eurydice. Only the War Hounds had proved able to resist the creatures' baleful influence, but massively outnumbered they had paid for their resistance in blood, and would have been overwhelmed regardless if huge numbers of enraged Orks, seemingly the massed forces of the entire Greenskin Glortian Empire, had not swarmed into the system.During the ensuing battle, the Orks fought with unbelievable fury even given their warlike species' tendencies, compelled by their baleful masters to exterminate their enemies. More of the gargantuan Psybrid hourglass voidships in turn had appeared to reinforce their fellows, and with them thousands of enslaved xenos warriors and ships, few of which were known even to the lexicanic data cores of the Imperial warships.Eurydice Terminal had become a killing star; a vortex of destruction that was even now calling more armies to their deaths, and here the Ultramarines had come to restart the battle afresh. Guilliman, having apprised himself of the tactical situation, modified his battle plans accordingly and without pause put his strategy of attack into operation.Despite Erad Krüg's request for his surviving Legionaries to be lifted from their wrecked vessel and given a place in the line of battle, the War Hounds were denied; this would be a battle for the Ultramarines alone. It was to be a tactical operation more traditionally the specialty of other Space Marine Legions than the Ultramarines, but at which Guilliman's own warriors were still well-versed -- a full assault-strike voidship boarding assault.The primarch's plan of attack was a shockingly direct one. His fleet didn't pause for a long-range bombardment or present a broadside and pound the enemy at close quarters; instead, he ordered his Legion to conduct a full boarding strike without prelude. Guilliman drove the core of his fleet and with it nearly 100,000 Ultramarines into the heart of the enemy.It was not without irony that Roboute Guilliman records in his own testimony about the assault that he and his Legion were adverse to strategies which resulted in a heavy cost of Ultramarines' lives; this single action would dispel that myth and illustrate the truth that such tactics were often wasteful and unneeded by a skilled general, but when such sacrifice was the most efficient and indeed perhaps the only path to victory, he and his Legion would pay that price with fervour and unbreakable determination.The Ultramarines at last unleashed their wrath as they closed with the enemy vessels, hundreds of gunships and assault rams roaring from their flight bays to the attack as the Ultramarines fleet unleashed a cannonade of a thousand lance batteries and macrocannons. The Psybrid nomad-vessels lashed about them with frenzied whips of elemental energy, and all around them hulls burst and ships burned, but it was too little against the unstoppable tide of ire which crashed against them.Through the maze-like networks of turning corridors like the innards of a great machine, the Ultramarines stormed with deadly intent, destroying as they went. Soon they encountered ghoulish, half-solid vapour forms, lean and gaunt without their exo-armour, who fought with the savagery of caged animals, burning bright with psychokinetic energy and striking out with mind-burning blasts of power, but for every Legionary who fell, a dozen pressed forward in their place and, one by one, the Psybrids began to be corralled, cornered and killed.Led by axe-and-shield-wielding Invictarii and moving bulwarks of Terminator Armour-clad Veterans, the Ultramarines pressed into the heart of the great vessels, mining the corridors and power junctions with Melta Bombs and atomantic implosion charges as they went. For the warriors of the XIII Legion were implacable, and in them had been woken a rarely displayed drive for vengeance against the Psybrids that could not be diverted nor denied.It was Roboute Guilliman himself who breached the innermost chamber of the largest of the hourglass ships to find the master of the Psybrids. It was a towering, multi-limbed gestalt-thing, thrice the primarch's own height, whose long, almost equine head screamed out shockwaves of ceramite-breaking force as its inner sanctum was invaded. But even as his own Terminator-armoured honour guard staggered and fell under the psychokinetic onslaught, the primarch, with his Chief Battle Psyker Aaroth Ptolemy beside him, charged.The Psybrid-king reared up above them, its clawed arms bearing strange weapons like the image of some forgotten devil-god of Old Night, but Guilliman with the strength and speed born of the Emperor's unmatched arts struck, cleaving the creature limb from vaporous limb as Ptolemy fought it on the psychic plane with every ounce of power in his possession, sacrificing his life so that Guilliman would be defended from the nightmarishly powerful creature's psychic assault.The monster finally fell and the primarch of the Ultramarines enacted his Legion's vengeance, ripping the thing's glowing brain from its skull and crushing it under his heel. One by one the towering hourglass vessels of the Osirian Psybrids fell, either torn apart by explosive charges from within, or rent to flinders in the crossfire of the waiting Ultramarines' echelons as they tried to break free. There was no escape.As the Psybrids died, so too did their slave armies still warring on the ground, and soon only the Glortian Orks, already mauled and all but exhausted, remained, and the Ultramarines made short work of driving them away once more into the outer darkness.The blood price of the battle had been high enough, several thousand Legionary casualties, and many among them veterans of his Legion's Terran roots, eager to be the first in battle in order to expunge the failing of the past. But it had been a price willingly paid for vengeance, and it would be a price and more that the Ultramarines Legion would willingly pay again in the future.