One of the earlier geopolitical entities to swear allegiance to the Emperor, the Achaemenid Empire had therefore suffered little during the Wars of Unification, avoiding both atomic strike and invasion by Thunder Warriors. A monarchical empire, the population were separated into tribal factions and were apparently a culture of truth and discourse. As a result of the relative peaceful nature of the region and the protection enjoyed by early adoption of Imperial allegiance, the Achaeminians sported few of the genetic defects or abnormalities seen amongst humans during this time and were therefore selected as a recruiting source for the infant Legiones Astartes. The Achaeminians provided the initial Terran intake assigned to Legion XV. The Achaeminians appeared to posses a form of superstitious belief in the mystical power of their ancestors; emblems of Dhul-Qarnayn, their greatest King, were used as charms against harm.
The name of the Achaemenid Empire draws its name from its historical Persian counterpart.