In Warhammer 40k, Phobos usually means the larger of Mars’ two moons: a gun-studded fortress that guards the Adeptus Mechanicus homeworld. It’s also the name of a separate Imperial world that raises Guard regiments, so it’s worth checking which one a source means.
What Phobos is
Phobos is one of Mars’ two moons, sitting in the Sol System in Segmentum Solar. It’s classed as a fortress world, and it’s covered in guns. Its job is simple: protect the Red Planet, capital of the Adeptus Mechanicus.
It’s not a hive or a shrine. Think of it as an orbital bunker complex, watched over by the Mechanicum and tied directly into Mars’ defence network.
- Primary: Mars
- System: Sol, Segmentum Solar
- Class: Fortress World
- Affiliation: Imperium, under the Adeptus Mechanicus
Vaults and the Horus Heresy
During the Great Crusade, Phobos held vaults of ancient and sometimes forbidden technology gathered by the Mechanicum. Only the Fabricator-General could authorise opening them.
When the Schism of Mars tore the Mechanicum apart in the Horus Heresy, loyalist Skitarii raided those vaults to keep the relics out of traitor hands. Later, during the Siege of Terra, surviving Loyalist Skitarii Conclaves launched a suicidal assault on the moon. They fought tech-horrors unleashed by Kelbor-Hal’s Dark Mechanicum, and still managed to cleanse Phobos while the Traitors were fixated on Terra.
The moon that moved
In M32, during the War of the Beast, Fabricator-General Kubik tested Ork teleportation technology by shifting Phobos from one side of Mars’ orbit to the other. Yes, they moved a moon to see if it would work.
The moon also has a manufacturing history. Phobos was producing boltguns from the late Unification Wars onward, which is why you’ll see the name attached to Phobos-pattern wargear.
The other Phobos
There’s a separate world also called Phobos, listed simply as an Imperial planet. It’s known for raising the Phobos Imperial Guard Regiments, including the Phobos 16th, which fought in the 13th Black Crusade.
The sources don’t place this planet in a named segmentum, so don’t assume it’s the moon. If a passage is about Guard regiments, it’s the planet. If it’s about Mars’ defences, vaults, or shipyards, it’s the moon.
| Name | What it is | Why it matters |
|---|---|---|
| Phobos (moon) | Fortress moon of Mars | Guards the Mechanicus homeworld, held relic vaults |
| Phobos (planet) | Imperial world, segmentum unknown | Raises the Phobos Imperial Guard Regiments |
Key facts
- Phobos is a gun-studded fortress moon orbiting Mars in the Sol System.
- It’s classed as a Fortress World and belongs to the Imperium under the Adeptus Mechanicus.
- Its vaults held ancient, sometimes forbidden technology, openable only by order of the Fabricator-General.
- Loyalist Skitarii raided those vaults during the Schism of Mars, and later cleansed the moon during the Siege of Terra.
- In M32, Fabricator-General Kubik teleported Phobos to the other side of Mars’ orbit while testing Ork teleportation tech.
- A separate Imperial planet named Phobos raises the Phobos Imperial Guard Regiments, including the 16th.
FAQ
Is Phobos a moon or a planet in Warhammer 40k?
Both names exist. Phobos is most often Mars’ fortress moon, but there’s also a separate Imperial planet called Phobos that raises Guard regiments. Context tells you which one is meant.
Why does Phobos matter to the Adeptus Mechanicus?
It’s an orbital fortress guarding Mars itself, and during the Great Crusade it stored ancient and forbidden technology that only the Fabricator-General could unseal.
What happened on Phobos during the Horus Heresy?
Loyalist Skitarii raided its vaults to stop relics falling to the traitors, and later a coalition of Loyalist Conclaves attacked the moon during the Siege of Terra, defeating Dark Mechanicum forces there.