The Diasporex was a nomadic, democratic confederation of Humans and aliens who lived aboard a vast collection of ancient starships, maintaining a network of solar collectors around various stars to fuel their vessels' power generators. The aliens who existed in alliance with the Diasporex Humans were a large quadrupedal species that lacked eyes and made use of cybernetic enhancements.
The Diasporex had been prowling the depths of space since at least the Age of Strife and their lifestyle may have evolved as a reaction to the horrors of that time on many Human-settled worlds. During the last days of the Great Crusade in the early 31st Millennium, the Iron Hands Legion, embedded in the 52nd Expeditionary Fleet, encountered the fleets of the Diasporex near the Carollis Star in the Lesser Bifold Cluster.
The Iron Hands offered the Human members of the Diasporex the opportunity to separate from their alien allies and to join the Imperium of Man. They declined, and so under standing Imperial orders had to be destroyed as a potential threat to the nascent Imperium due to their alliance with xenos.
In the solar months that followed, the Iron Hands Legion fleet tried to destroy the Diasporex, but their fleets proved to be highly skilled and experienced in naval warfare: they easily evaded multiple fleet engagements initiated by the Astartes and even managed to damage an Iron Hands cruiser.
At this point, the Emperor's Children Legion and their 28th Expeditionary Fleet were called in as reinforcements for the Iron Hands. The combined Astartes force eventually made the discovery that the Diasporex used hidden solar collectors positioned around the Carollis Star to collect fuel for their starships, which was the reason why they had remained in that region of space rather than just fleeing from the Imperial forces.
The Space Marines assaulted these solar collectors around the Carollis Star and thus forced the Diasporex fleet into open battle to protect them, where they were defeated. The Diasporex's democratic society proved their undoing, as they found it more difficult to coordinate an effective defence than the centrally-commanded Space Marine force of the Imperium.
A brief shipboarding action occurred that ended this Battle of the Carollis Star, in which the Emperor's Children 2nd Company Captain Solomon Demeter stole the glory of seizing the bridge of the Diasporex command ship from the increasingly corrupted Primarch Fulgrim.
The final words of the alien psyker that served as the Diasporex command ship's captain, one of the quadrupedal xenos who were allied with the Humans, were telepathically broadcast to Solomon Demeter.
"All we wished was to be left alone."
In the wake of the Imperial victory, the surviving Human Diasporex prisoners were handed over to the planetary governor of the closest Compliant world to be used as slave labour. The quadruped aliens were simply executed, their vessels destroyed by the Imperial fleet.
A detachment of the Mechanicum from the 28th Expeditionary Fleet was left behind to study what remained of the ancient Human technologies of the Diasporex, and Fulgrim gave them leave to rejoin the 28th Expedition upon the completion of their researches.
- Fulgrim (Novel) by Graham McNeill, Chs. 6-11