The Osiris Cluster was initially forced into compliance with the early Imperium by the XIIIth Legion in a highly successful campaign. However suddenly, the region erupted into open rebellion. The Legion's commanding officer at this time, Lord Commander Gren Vosotho, acted swiftly and deployed for the Cluster. The primary targets of the Imperial forces in the suppression operation was the world of Septus XII and in particular its Hive city of Cabasset.
The attack began well enough, with Vosotho personally leading the airborne 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. It became apparent this was no ordinary enemy, but the legion nonetheless slowly advanced into the Hive City. It was then, when the XIIIth Legion was heavily committed and kilometres deep into the hive, that the trap was sprung. A xenos fleet of unknown type and origin, comprising five vast hourglass-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. Attacked in conjunction by the human rebels with an even more fanatic fervor, The 12th Expeditionary Fleet was overmatched but fought on valiantly, causing one of the titanic xenos craft to fall back. It was then that the xenos began teleporting directly on the battlefield and on Imperial warships. They were Armoured in some form of baroque bio-mechanical containment suits, the creatures within were barely corporeal; 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. 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 Storm bird from the ground assault had departed the planet. Ultimately, the XIIIth had lost 6,500 Astartes in the disastrous battle, the bloodiest of any of their campaigns in the Great Crusade. When the XIIIth returned a year later with heavy Imperial reinforcements, they discovered the worlds to be a barren wasteland with either death or the survivors locked in civil war. Of the mysterious xenos, there were no signs. 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 certain 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.