Luna is Terra’s single moon, and in Warhammer 40k it’s much more than a rock in the sky. It’s the birthplace of the Space Marines, the Imperium’s first conquest in the Great Crusade, and a bristling fortress that guards the Throneworld itself.
What Luna actually is
Luna sits in the Sol System, in the Segmentum Solar, orbiting Terra. It’s classified as a civilised world, a death world, and an industrial world depending on which source you read, which tells you a lot about how rough life there can be.
Billions of people live on it, mostly underground in hive cities. The surface is dotted with factories and defence structures, and large stretches of it are still barren. There’s a thin atmosphere and artificial gravity, but you wouldn’t call it comfortable.
- Segmentum: Solar, Subsector Sol, Sol System, primary Terra
- Affiliation: Imperium
- Classification: civilised, death, and industrial world (sources vary)
- Population: billions, largely underground
The gene-labs and the birth of the Legiones Astartes
During the Dark Age of Technology, Luna was a centre of human genetic research, home to vast underground laboratories. At its ancient peak it out-produced Terra in industry, shipbuilding, and army production.
By the Age of Strife, fanatical Selenar gene-cults ruled the moon. The Emperor broke them in the First Pacification of Luna, the opening battle of the Great Crusade. Taking those labs let him mass-produce the Legiones Astartes, which is why Luna matters so much in the setting’s history.
As the Crusade pushed outward, Astartes production on Luna faded and the Selenar lost their influence. The moon was handed over to the Sisters of Silence, Departmento Munitorum facilities, and Imperialis Armada defences.
The Horus Heresy and after
During the Solar War phase of the Horus Heresy, Luna was hit hard. Abaddon led a major traitor assault, a Battle Barge called the War Oath rammed its orbital defence ring, and the traitors seized the remaining gene-labs.
The Selenar cults, still bitter at the Imperium, sided with Horus and shared their gene-tech with him. After the traitors lost the Siege of Terra, Luna was retaken in the first battle of the Great Scouring and repopulated with hundreds of thousands of imported Terrans.
Luna in the 41st millennium
By M41, Luna has decayed. Terra has long since squeezed it dry, and much of the population is bitter, blaming Terra for exploiting and then forgetting them. Outside the main port facilities, infrastructure has rotted and many people live in squalor.
The Somnus Citadel is a good example: the Imperium thought it abandoned. After Cadia fell, Luna became a battlefield again during the Terran Crusade, when Magnus and the Thousand Sons emerged on the moon as Roboute Guilliman tried to reach Terra through the Webway.
Key facts
- Luna is Terra’s only moon, in the Sol System, Segmentum Solar
- It was a genetic research hub during the Dark Age of Technology
- The First Pacification of Luna was the Great Crusade’s opening battle
- Seizing its gene-labs let the Emperor mass-produce the Legiones Astartes
- It was attacked by Horus’s forces during the Solar War and retaken in the Great Scouring
- By M41 it’s a decaying, resource-stripped world with a bitter population
FAQ
Why is Luna important in Warhammer 40k?
It’s where the Emperor seized the Selenar gene-labs and gained the means to mass-produce the Legiones Astartes. It’s also the fortress moon guarding Terra, so it’s strategically vital to the Imperium.
What happened to Luna after the Horus Heresy?
It was retaken by loyalists during the Great Scouring and repopulated with Terrans. Over the following millennia it decayed, its resources stripped by Terra, and much of its population ended up living in poverty.
Who were the Selenar gene-cults?
They were the fanatical gene-cults that ruled Luna during the Age of Strife. The Emperor defeated them in the First Pacification of Luna, and they later sided with Horus during the Heresy out of resentment.