Luna

Luna IRL.

Luna, or “The Moon”, is a natural satellite of Earth blasted with craters. In fantasy stories, the Moon is associated with creatures of the night such as vampires and werewolves. In sci-fi stories it's often one of mankind's first space bases, such as in 40k where it's a large naval dock.

As of 2023 it is the only other celestial body which humans have set foot on.

IRL

The Moon, being Earth's eternal companion, has been watching over us since we were just hunter-gathers who just figured out this whole 'walking on two legs and using tools' business - and our evolutionary ancestors before that. As a result, the Moon is an ingrained part of humanity's collective culture, capturing our imaginations since before the dawn of history. In modern times it's the celestial body other than the Earth that we know the most about, and as mentioned previously the one we actually managed to walk around on.

Currently, science believes that Luna came into existence by the death of another body shortly after the formation of the Earth itself. ~4.5 billion years ago, the latest collision in a cataclysmic game of cosmic billiards that was the Solar System forming would involve young Terra and a Mars-sized protoplanet dubbed Theia. This giant impact would see Theia destroyed, and Earth survive (though in what state is still debated, though it definitely extended the 'primordial hellscape' phase) - debris from the corpse as well as Terra would get trapped in orbit and coalesce into the Moon. Besides making the Moon (and causing phenomena related to it), Theia's crash-and-burn is also the most likely culprit for what gave Earth its axial tilt, so if you live somewhere with four seasons, this is why.

Since then, the Moon's become tidally locked, hence why there's a "dark side" that faces away from us. It's also drifted away significantly, which it continues to do so at a snail's pace. Finally, ever since its formation, the Moon has been responsible for stabilizing the Earth's tilt so it doesn't drift significantly (which prevents having the same wild temperature swings as say, Mercury), as well as creating tides.

The Moon's distinctive look is caused by the fact it has no atmosphere, so any meteor that gets caught on a crash course will impact the surface. Smaller major impacts cause the large craters on the Moon, while the largest ones back in the day made Luna vomit up its insides and create giant oceans of lava which we call 'lunar mare' long after they hardened into rock.

Warhammer 40k

Luna was a vital defensive fortification/space port for the Imperium of Man that served as a launching pad for the Great Crusade due to the vast genetic research laboratories and gene seed factories that allowed the Emperor to mass produce the Space Marine legions, and ultimately give the majority of the galaxy a righteous secular fisting. It was a battleground for two notable conflicts; The First Pacification of Luna which kicked off the Great Crusade, and the Solar Wars which was the site of a major attack by the traitor legions during the Horus Heresy.

The Early Years

Sometime during the Dark Age of Technology, Luna was taken over by a gene cult known as the Selenar, a super secret order that existed since antiquity and consistently changed its face by latching onto different religions cropping up on Terra until they were able to, as any evil fanatical organization dreams of, make it to the moon. Their main goal of keeping the power of creation out of the hands of humanity, and driven beliefs of some bullshit involving humans being fractal transcendent beings acting as echoes of archetypes found throughout humanity, spurred them into creating sprawing genetic research facilities underneath Luna in pursuit of the understanding of human nature. Millennia of obsessive experimentation resulted in their magnum opus, the Magna Mater; AKA the Sangprimus Portum. The Selenar also had a habit of cloning successive copies of themselves to persist through time and continue their projects, probably so they wouldn't have to relinquish oversight to somebody not them.

The First Pacification of Luna

Eventually, Big E was looking towards the sky for his next conquest with his sights on Luna. His first attempts of diplomacy ended up with his emissaries returning as a screaming soup of molten metal and flesh, which he responded in kind with a 6 hour shit-kicking in from three deployed space marine legions.

Legion XVI lead the assault as acting vanguard in Terra-based rockets, landing on the moon's surface under fire from the gene cult's ground base defenses while sneaking in assault vehicles aided by aerial distractions from the XIII and VII legions. Legion XVI descended underground into the subterranean complexes, reducing the half-machine genetic abominations lumbering about into mush with bolters, obliterated resistance points in seconds, stormed the orbital defense center shutting everything down and leaving a hundred Selenar dead before they even realized they'd been compromised. The other two legions swept in, and the planet's forces crumbled in the face of superior firepower and systematic annihilation. The Selenar matriarch Heliosa-54 offered surrender, begging the Emperor to "call off his wolves" (HA). Emps let the remaining Selenar gracious live, took the Magna Mater despite their best attempts to hide it, and utilized it as the source code to draft the genetic templates of the Primarchs and mass produce the now modern variant of the Space Marines with his fancy new gene labs. In remembrance of the battle, the Emperor dubbed Legion XVI as "the Imperial Wolves", and later named them "the Luna Wolves".

Towards the later expansion of the Great Crusade, Luna, along with the Selenar, fell out of prestige as the Imperium's premier space boy maker, and was subsequently handed over to the Sisters of Silence, Departmento Munitorum and the Imperial Navy to develop into a giant dockyard and planetary defense station.

The Solar Wars

As Luna became essentially a glorified ball of guns and ships, it was a prime defensive line grouped in with Terra as the Fifth Sphere commanded by Rogal Dorn against the onslaught of traitor legions near the end of the Horus Heresy. It's massive orbital ring held more firepower than entire expeditionary fleets which could tear several new assholes in anything ballsy enough to get close.

A contingent of the Sons of Horus appeared over Luna, headed by First Captain Abbadon. Realizing his minuscule force was hovering directly over a whirling cheese grater of ballistic supremacy, he proceeded to ram the fuck out of Luna's orbital ring with his battle barge War Oath, blowing a 20km hole straight through it, and teleport his crew to the surface. Justaerin and Word Bearers assault forces joined in through the gaping hole, and they proceeded to capture the remaining gene-labs under Luna with the same ferocity once shown in the First Pacification.

Rogel Dorn gave strict orders to the Selenar, still toiling away at genetic research in the labs, to destroy all gene-tech to keep it out of the traitor legions hands. Turns out forcing them at gunpoint to labor for free grew resentment towards the Emperor, and they freely sided with Horus because fuck the Imperium. One of the Selenar was somehow still protecting the Magna Mater, disobeyed her Matriarch and sent out a distress signal, received by a loyalist vessel Sisypheym, and collected during a raid on Luna forever denying the Chaos chapters of Chadmarines. Luna eventually was recaptured during the Great Scouring.

In the Present

Luna in 41M is basically Detroit, run down and decayed after it's resources stripped and the one thing it was good at becoming meaningless, with most of the population hating Terra for being wrung dry and forgotten. Outside of a few key facilities, the rest of the moon is abandoned and falling apart, people living in poverty and squalor.

After the fall of Cadia, Luna once again saw Chaos on its surface, but luckily with the reawakening of Roboute Guilliman, was reinforced by Imperial Fists, Custodies, Battlefleet Solar and the Sisters of Silence. Now that the Indomitus Crusade is underway, Luna began a reformation under Guilliman and is beginning to return to former glory as a fortress world for the Sisters of Silence again.