Luna is the traditional name for the moon of Terra which the Imperium of Man classifies as both a Dead World and a Civilised World that is home to billions of people in hive cities built below the surface of the lunar regolith. Humans first landed on Luna in the year 969.M2, making it the first world ever explored by Mankind in the ancient past. In the subsequent centuries, it was one of the first worlds colonised by humanity, who erected great domed cities above and below its airless surface.
Luna became a centre of scientific research and zero-gravity manufacturing during the Age of Technology, when it was the home of the Selenar gene-cults who saw the perfection of advanced genetic engineering techniques as a necessary component of a faith similar in some ways to that of the Cult Mechanicus of the Mechanicum of Mars. The Emperor of Mankind conquered and united Old Earth during the Unification Wars following the Age of Strife in the middle of the 30th Millennium and then sent His forces to occupy the human colonies on Luna.
It was in the lunar gene-laboratories of the Selenar that the Emperor began the Primarch Project, the massive genetic engineering research initiative necessary to create the Primarchs from the foundations of His own genome. However, the Primarchs' actual gestation was carried out in his secret gene-laboratories beneath the Himalazian (Himalyan) Mountains to provide greater protection from the interference of the Dark Gods of Chaos.
Unfortunately, these arcane protections ultimately proved inadequate and the Primarchs, still developing infants locked into their gestation capsules, were scattered across the galaxy after the Ruinous Powers opened a Warp Rift within the Imperial laboratories. Yet from the Primarchs' remaining genetic material the Emperor was able to create the first Space Marine Legions, using the organs cultured from the Primarchs' gene-seed to transform ordinary human men into transhuman warriors.
Luna was Terra's only natural satellite. Although not the largest natural satellite in the Segmentum Solar, it is the largest relative to the size of the planet that it orbits. Luna is in synchronous rotation with Terra, always showing the same face with its near side marked by dark volcanic maria that fill between the bright ancient crustal highlands and the prominent impact craters.
Once, the satellite had been a mottled stone wasteland where humans had ventured in their first infantile steps away from their birth world. They had built colonies there, testing their mettle in the pitiless cold of the void in preparation for future voyages to other planets, but as Terra's people had advanced, Luna had become little more than a way station, a place to pass by on the journey to the interplanetary -- and later, interstellar -- deeps. This age of exploration and technology came to an end with the coming of Old Night, the chaotic and highly fragmented period of human history between the 25th and 30th Millennia.
By the end of the Age of Strife, Luna had become the bastion of a conglomeration of resurrectionist gene-cults known as the Selenar, whose members believed that human nature was both fractal, fractured and transcendent. To them, the true nature of Mankind was visible in the flesh and mind of the individual, but each person was an echo of archetypes that persisted throughout human existence. Each Selenar gene-cult clung to a different set of archetypes: some represented them as anthropomorphic figures, some as numbers, and some as symbolic systems.
Shot through with low occultism, most fell short of deification, but all pursued understanding through their craft of gene-forging. Each cult member was a product not of random breeding, but creation by the cult's gene-wrights according to formulae born in the Dark Age of Technology. Resurrected in body time and time again, they attempted to reach a true personification of a single human archetype. In their subterranean complexes, the cults were powerful, insular and resistant to the Imperial Truth. Such resistance was hardly a novelty, and would normally have been dealt with in the usual manner -- by obliteration. The fact that the gene-cults had something that the growing Imperium needed complicated that position.
Clouded as it might have been by occultism, the gene-craft of the Selenar Luna cults was unequalled save within the Emperor's own facilities on Terra. With Terra teetering on the edge of complete Unification by 798.M30, the conquest of the Sol System and the galaxy beyond beckoned. For the Great Crusade there would need to be more and more Space Marines. The Terran gene-engineering facilities would not be able to cope with such a huge increase in the creation of recruits for the Space Marine Legions, so the newborn Imperium of Man needed Luna's wealth of gene-craft and technological capacity added to its own.
But the Selenar gene-cults would not bow willingly to the hunger of the new-born Imperium, and had fought off the machinations of Terran warlords and Martian Magi in the past. The Selenar had received Imperial ambassadors with courtesy, but always answered their offers, threats and entreaties with silence. As the threats began to outnumber the offers, the Luna cults began to turn away the ambassadors and gird themselves for war. Normally highly factional, the possibility of Terran domination overcame all doctrinal disagreements between the Selenar cult matriarchs and gene-wrights -- they would resist the Imperium together and without limit.
The final Imperial deputation to Luna was returned to Terra as a still-screaming soup of fused liquescent flesh in the dead shell of their shuttle. 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. As such, Luna would become the site of the first off-world Imperial Compliance action ever undertaken by the early Astartes Legions, even while the Unification Wars were still raging on Terra before the year 800.M30.
First Pacification of Luna
To the task of conquering Luna, the Emperor set 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 combined force of the VII, XIII and XVI Legions (later named the Imperial Fists, Ultramarines and Luna Wolves) lifted from the surface of Terra in a scattering of rocket flame. The as-yet-unnamed XVI Legion had been chosen to serve as this Space Marine force's vanguard, and had brought its full strength to bear.
Cutting power to their assault craft, the Astartes of the XVI Legion drifted silently towards Luna from Mankind's birthworld through the void like arrows fired into the night. As the smaller wave of assault craft belonging to the VII and XIII Legions approached the airless world, the Selenar defensive weapon systems embedded in Luna's surface lashed the oncoming Imperial force. The XVI 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, their surrender communique 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 XVI Legion, they had earned their name -- the Luna Wolves.
For a time in the Age of Strife when Terra was engulfed in war and blood, the Moon had become desolate and empty once again despite the small population of Selenar, but after the rise of the Emperor and its conquest by the Imperium, Luna knew a rebirth. Waxing and waning, the satellite came full circle as the Age of the Imperium brought it new life.
Bisecting the grey stone sphere across its equator lay a man-made valley many kilometres wide. This was "the Circuit," an artificial canyon that laid open the rock and stone beneath the dusty lunar surface. All along the length of the chasm lay gateways into the Moon's interior, vast doors to the honeycomb of spaces carved by Mankind in the heart of Luna. The ancient, dead boulder of the Moon became the largest military complex ever built by humans. A vast orbital shipyard for the armada of the Imperium, thousands of starships from the smallest shuttle to the largest Battle Barge, were built and maintained there, and across the face of the Far Side there were complex stations for observation of the great void beyond.
Port Luna was the cold, stone heart of humankind's great fleets. The satellite was as much a weapon as it was a safe harbour. Much of the metals mined from the Moon's heart and the rock from the Circuit's excavation had been employed by the Emperor's most-skilled engineers, fashioned into a synthetic ring that girdled the planetoid. The vast, grey hoop held batteries of Lance cannons and docking bays for more warships. Wherever the light from Luna fell, those who saw it could sleep soundly knowing the ceaseless guardian stood to their defence. And beyond it lay the sacred soil of Terra itself.
During this period in the era of the Great Crusade, Luna was home to the Sisters of Silence, known also as the Silent Sisterhood. They were the militant arm of the Adeptus Astra Telepathica and were internally referred to as that organisation's Departmento Investigates. Their base of operations, the mighty brass fortress-monastery known as the Somnus Citadel, was located on Luna. Within the mighty fortress were multiple tertiary landing ports designed for shuttles deployed aboard the Black Ships, the dread starships that make up the fleet of the League of Blackships, which forms the recruiting division of the Adeptus Astra Telepathica.
The Gene-Seed Crisis
As the Emperor's Wars of Unity broke the bounds of Terra, the pacification of the gene-cults of Luna and the Martian Compact allowed the Imperium to produce and equip new Space Marines at an unprecedented rate, and the Legiones Astartes began to expand to meet the demands of war across the stars. It had been apparent since the beginning of the Great Crusade that the Emperor's great vision of a united humanity free of the shackles of torment wrought by a nightmarish cosmos was threatened by as many enemies within the Imperium as without. No clearer sign that such hidden foes could strike at the heart of the Imperium occurred than the so-called "accident" that resulted in the gene-seed crisis that was nearly to doom the III Legion (the future Emperor's Children) -- an act which many have since been attributed to the actions of a secret enemy within.
As the gene-forges began to implant gene-seed organs into recruits for all the Legions, a portion of the III Legion's gene-seed reserve was despatched to Luna for establishment there. What happened next is not clear. Some claim the elements of the Selenite cults still resistant to the Imperium hijacked a Defence Laser and destroyed the ship carrying the III Legion's gene-seed, while conflicting accounts recount that the ship lost control and crashed as it was attempting to dock, whilst others claim that it simply vanished.
It would be too easy, in retrospect, to assign hidden conspiracy or malignant agency to this disaster, though this is not a possibility that can be discounted. The loss of the III Legion's gene-seed reserve was a severe blow, but it would not have endangered the Legion's survival if a second calamity had not occurred in quick succession.
The Horus Heresy
During the latter years of the great rebellion known as the Horus Heresy, the Warmaster Horus, first amongst Primarchs and most-favoured son of the Emperor, led his Traitor Legions to the very heart of the Imperium. Possessed as he was, the Warmaster had lost none of his strategic bluntness: crush the heart, and the Imperium could be remoulded as he saw fit in his own warped image. Striking from Dark Mechanicum-controlled Mars, the Traitor ships destroyed the Sol System's Imperial military bases, including those on Luna, smashed Terra's orbiting defences and scattered the protecting Loyalist fleets.
The Imperial fleet, or at least those vessels of it that remained loyal, was dispersed and leaderless, and succeeded only in driving a few rebel warships from the Sol System before suffering such heavy casualties that they were blockaded at the lunar bases. All across the galaxy many of the Emperor's greatest works had fallen and His forces were in disarray, having either been corrupted by Chaos or locked in bloody conflict with the servants of the Chaos Gods.
Such was the position when the forces of Horus gathered around Terra. The lunar bases, bastion of the Battlefleet Solar, fell to Horus following brief but hard fighting, and soon the Traitor fleet was in control of the orbital space around Terra. The Battle of Terra then began in earnest, with all of its consequences for the Imperium and Mankind's future.
Luna is home to immense planetary Defence Lasers charged with protecting Terra from invasion that are constructed on an orbital belt of defensive platforms that encircle the circumference of the world and is known as "The Ring." Made from rock excavated from Luna itself, The Ring is bristling with countless Lance, torpedo, and Macrocannon batteries.
These defences inflicted savage losses on the invading Traitor Legion fleets of the Warmaster Horus during the climactic stages of the Horus Heresy. After the Horus Heresy, Luna continued to serve as a massive orbital shipyard and the home of immense planetary Defence Lasers intended to serve as one of the final lines of defence for Terra.
Luna was the home to the greatest naval base in the Imperium of Man at the time of the Great Crusade and the Horus Heresy, known as Port Luna, where Battlefleet Sol was based. It was created from an artificial ring that had been positioned into orbit around Luna after the Imperial conquest composed of metals mined from the Moon's heart and the rock from the Circuit's excavation by the Emperor's most-skilled engineers. Due to the number of Imperial starships constructed at Port Luna, Luna also gave its name to the most common class of Imperial Navy Cruiser, the Lunar-class.
"The Circuit" was the name given to a massive artificial valley that had been gouged into Luna's surface that was home to countless military compounds, command centres, manufactoria, and other facilities designed to aid the war efforts of the Imperium.
- Codex: Space Marines (4th Edition), pp. 13-14
- Realms of Chaos: Slaves to Darkness (1st Edition), pg. 241
- The Horus Heresy - Book One: Betrayal (Imperial Armour) by Alan Bligh, pp. 80-81, 104
- The Horus Heresy - Collected Visions, pg. 354
- The Flight of the Eisenstein (Novel) by James Swallow, Ch.15