Battle of the Eurydice Terminal
Seeking to erase the shame at the heavy Ultramarines losses at the hands of the Osirian Psybrids in the Osiris Rebellion, Roboute Guilliman led a hunt for the sinister and mysterious Xenos. After many cold trails, they finally were able to confront the Psybirds at the Eurydice Terminal after reports that the War Hounds under Erad Krug had confronted Psybird-style vessels while on campaign against Orks of the Glortian Empire. The Ultramarines rushed to the region to extract their revenge.
When the Ultramarines arrived, the entire System had been reduced to a radioactive battleground. Seven of the Psybrids vessels were present in the system, under frenzied attack from Orks, as were several hundred surviving War Hounds on the Battle Cruiser Cold Cerberus. The War Hounds informed the Ultramarines that the Psybirds managed to mentally enslave a group of Feral Orks and were seeking to do the same to the Imperial defenders of Eurydice. The Ultramarines aided their War Hounds comrades, but insisted that the continuation of the battle would be theirs alone.
Having long studies the Psybrids, Guilliman knew to focus his attacks on the aliens themselves as opposed to their enslaved forces. Launching a swift boarding operation against the alien vessels, Guilliman himself led the attack and amid a fury of close-range Macrocannon attacks and boarding ships the Ultramarines advanced into the alien vessels. The ghoulish, half-solid vapor forms of the Psybrids, lean and gaunt without their exo-armour, fought with the savagery of caged animals. Invictarii Terminators led the assault inside the alien vessels. 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. Guilliman was protected by the creature's psychic abilities thanks to the sacrifice of Ptolemy, allowing the Primarch to rip the xenos to pieces.
One by one the towering hourglass vessels of the Osiran Psybrids fell, and as the creatures died so did their slave armies. Nonetheless Legion casualties were heavy, at least several thousand.