The First Pacification of Luna represented what is considered by some Imperial savants as the first known off-world campaign of the Great Crusade.
The Selenar gene-cults who called Luna home during the Age of Strife and the Unification Wars possessed highly advanced genetic engineering technology that would be of great use to the preparations for the Emperor's Great Crusade.
Despite repeated attempts by the Emperor to bring Luna into the nascent Imperium of Man through peaceful diplomatic means, the gene-cults refused all offers. This required the Emperor to unleash three of the early Space Marine Legions upon Luna to bring the world into Imperial Compliance.
This early Imperial campaign was the first operation mounted by elements of the early Space Marine Legions beyond the skies of Terra. The Unification Wars were still raging across the surface of Mankind's birth world at this time.
Luna was the bastion of a conglomeration of resurrectionist gene-cults whose members believed that human nature was both fractal, fractured and transcendent. Each of these Selenar gene-cults clung to a different set of archetypes. Every cult member was a product of creation by the Selenar gene-wrights according to formulae crafted in the Dark Age of Technology. Resurrected in body time and time again they sought to distill the true personification of a single human archetype.
In their subterranean complexes the cults were powerful, insular and resistant to the Imperial Truth. The Imperials would have normally dealt with these insular cults in the usual matter they handled all of the other Terran factions and techno-barbarian states that refused to accept the rule of the Emperor of Mankind -- by obliteration.
The fact that the Selenar gene-cults had something that the growing Imperium needed complicated that position. The Selenar returned the Imperial entreaties for alliance with silence. As the threats of the Imperium soon began to outnumber its offers, the Selenar cults began to gird themselves for war.
So it was that the Emperor finally ordered Luna to be pacified by the sword, their superstitious beliefs cast down before the Imperial Truth and their gene-craft yoked to the needs of the Imperium. To this task the Emperor set the three of His newborn Space Marine Legions most suited to this purpose on what some Imperial chroniclers name as the first true battle of the Great Crusade.
The as yet unnamed XVIth Legion had been chosen to serve as the Space Marine force's vanguard, and had brought its full strength to bear. Cutting power to their assault craft, the Astartes of the XVIth Legion drifted silently towards Luna through the void like arrows fired into the night.
As the smaller wave of assault craft belonging to the VIIth and XIIIth Legions approached the airless world, the Selenar defensive weapon systems embedded in Luna's surface lashed the oncoming Imperial force. The XVIth Legion's assault craft, unlooked for and unseen, struck their targets like a dagger in the night.
Within six solar hours of the first shot being fired, Luna had been pacified and brought into Imperial Compliance, the first off-world conquest of the Imperium of Man. Faced with annihilation, the surviving Selenar cultists bent the knee instead, the surrender communique of High Matriarch Heliosa-54 transmitted to Terra calling for the Emperor to "call off his wolves."
Broken and humbled, the enslaved gene-wrights of Luna would help forge the next generation of Space Marine who would carry out Mankind's conquest of the stars. As for the XVIth Legion, they had earned their name -- the Luna Wolves.
- The Horus Heresy Book One - Betrayal (Forge World Series) by Alan Bligh, pp. 80–81
- Siege of Terra: Sons of the Selenar (Novella) by Graham McNeill, Prologue