A Space Marine Legion was one of the twenty original Legiones Astartes, the massive armies the Emperor raised during the Great Crusade to reunite humanity. Each was led by a Primarch and could field tens of thousands of Marines, making them far larger than the Chapters that replaced them after the Horus Heresy. They’re the foundation of everything the Adeptus Astartes became.
What a legion actually was
The Legions were the Emperor’s primary military force during the Great Crusade, founded to bring scattered human worlds back into the fold. Each of the twenty was raised from gene-seed tied to a specific Primarch, which gave them distinct cultures, tactics, temperaments, and physical traits.
Unlike later Space Marine Chapters, a Legion operated as one huge organization under a single commander. They had their own fleets, armoury, and support structures, and they answered directly to the Emperor or their Primarch. That scale let them conquer entire worlds, but it also made them dangerous when a Primarch turned traitor, because he took his whole Legion with him.
Legions recruited from specific worlds, often the Primarch’s home planet or worlds they’d conquered. Cthonia supplied the Sons of Horus, Chemos the Emperor’s Children, and so on. These recruiting grounds shaped each Legion’s culture. Gene-seed came from the Primarch’s genetic template, and it’s what made a Marine more than human: extra organs, heightened senses, and the resilience to survive wounds that would kill anyone else. Each Legion’s gene-seed had quirks, and some proved more stable than others.
The twenty Legions
The Emperor created twenty Legions, but two were lost before the Horus Heresy. Their names and records were expunged, and nobody in-universe talks about why. That leaves eighteen known Legions, nine of which stayed loyal to the Emperor and nine that turned against him during the Heresy. Each Legion had a number, a name, and a Primarch. Some are famous, others were destroyed or forgotten before the Heresy even began.
The loyalist Legions were later broken into smaller Chapters, while the traitor Legions fled into the Eye of Terror and continued as large, often daemon-infused warbands.
| Legion | Primarch | Allegiance |
|---|---|---|
| I – Dark Angels | Lion El’Jonson | Loyalist |
| III – Emperor’s Children | Fulgrim | Traitor |
| IV – Iron Warriors | Perturabo | Traitor |
| V – White Scars | Jaghatai Khan | Loyalist |
| VI – Space Wolves | Leman Russ | Loyalist |
| VII – Imperial Fists | Rogal Dorn | Loyalist |
| VIII – Night Lords | Konrad Curze | Traitor |
| IX – Blood Angels | Sanguinius | Loyalist |
| X – Iron Hands | Ferrus Manus | Loyalist |
| XII – World Eaters | Angron | Traitor |
| XIII – Ultramarines | Roboute Guilliman | Loyalist |
| XIV – Death Guard | Mortarion | Traitor |
| XV – Thousand Sons | Magnus the Red | Traitor |
| XVI – Sons of Horus | Horus Lupercal | Traitor |
| XVII – Word Bearers | Lorgar Aurelian | Traitor |
| XVIII – Salamanders | Vulkan | Loyalist |
| XIX – Raven Guard | Corvus Corax | Loyalist |
| XX – Alpha Legion | Alpharius (and Omegon?) | Traitor |
From legions to chapters
The Horus Heresy broke the Legions apart. Afterward, Roboute Guilliman wrote the Codex Astartes, which split the surviving loyalist Legions into smaller Chapters of roughly a thousand Marines each. The goal was to prevent any single commander from wielding the kind of power that had let Horus rebel.
Not every Legion followed the Codex strictly. The Space Wolves kept their own structure and stayed larger than a standard Chapter, while the Black Templars ignored the size limit entirely. The traitor Legions never split, though their warbands often operate independently.
So when you hear ‘Chapter’ today, that’s the post-Heresy version. The Legions are the original thing, and they’re gone as unified loyalist forces.
Why the distinction still matters
In modern Warhammer 40,000, the word legion carries weight. It marks a lineage back to the founding twenty, and it separates the original organizations from the Chapters that came after.
For players, legion identity often determines paint schemes, rules, and lore. For the setting, it’s the fault line running through the Imperium: the moment when brotherhood turned into betrayal.
Key facts
- The Legiones Astartes were the twenty original Space Marine Legions.
- Two Legions were lost and expunged from Imperial records, leaving eighteen known.
- Each Legion was led by a Primarch and could number in the tens of thousands.
- Nine Legions stayed loyal during the Horus Heresy, nine turned traitor.
- Loyalist Legions were split into Chapters after the Heresy; traitor Legions were not.
- Gene-seed from each Primarch gave the Legions distinct traits and cultures.
FAQ
How many Space Marine Legions were there?
Twenty were created, but two were lost and erased from Imperial records. That leaves eighteen known Legions, nine loyalist and nine traitor.
What’s the difference between a Legion and a Chapter?
A Legion was a huge, unified army under a Primarch, often numbering tens of thousands. A Chapter is a smaller, self-contained force of about a thousand Marines, as set out in the Codex Astartes.
What happened to the two lost Legions?
Nobody knows in-universe. Their records were wiped and it’s never explained. It’s one of the setting’s deliberate mysteries.
Do the traitor Legions still exist?
Yes. They fled into the Eye of Terror after the Heresy and continue as warbands or, in some cases, still organized Legions dedicated to Chaos. The loyalist Legions became Chapters and no longer exist as Legions.