The First Founding is the moment the Emperor created the original twenty Space Marine Legions, the template for every Chapter that’s followed. Nine of those Legions stayed loyal, nine turned traitor during the Horus Heresy, and two were erased from Imperial records entirely.
What the First Founding actually was
After the Age of Strife nearly finished off humanity, the Emperor needed an army that could reunite the scattered worlds of mankind. He made the Space Marines: warriors engineered from his own genetic material, organised into twenty Legions. That’s the First Founding.
Every later Founding is a copy of a copy. The First Founding Legions were the originals, built directly from the Emperor’s gene-seed and led by the Primarchs, his genetically crafted sons. When the Great Crusade rolled out across the galaxy, these twenty Legions were the spearhead.
The Horus Heresy broke that unity. Half the Legions followed Horus into rebellion, and the loyalists who survived were eventually broken down into smaller Chapters under the Codex Astartes. So the First Founding is both the beginning of the Space Marines and the source of the Imperium’s oldest grudge.
The loyalist nine (and the rest)
Of the twenty original Legions, nine stayed loyal to the Emperor. Those nine are the ones the Imperium celebrates as the First Founding Chapters today, and they’re the focus of Black Library’s 2023 background book First Founding.
The other nine Legions turned traitor. Two more, the II and XI Legions, were purged from history entirely, their Primarchs and records scrubbed by the Emperor himself. Nobody in-universe agrees on why, and the sources deliberately never say.
- Dark Angels
- White Scars
- Space Wolves
- Imperial Fists
- Blood Angels
- Iron Hands
- Ultramarines
- Salamanders
- Raven Guard
Why the First Founding still matters
In the Era Indomitus, those nine loyalist brotherhoods are still fighting. Once Legions, now Chapters, they’re the oldest and most storied formations in the Adeptus Astartes. Their gene-seed lines, rivalries, and doctrines shape how the Imperium wages war.
It’s also the root of most Space Marine lore you’ll run into. Successor Chapters trace their lineage back to one of the nine loyalist Legions (or claim to). Heresy-era grudges, like the Space Wolves and Thousand Sons, or the Dark Angels’ obsession with the Fallen, all start here.
The traitor Legions matter just as much. They became the core of the Chaos Space Marines, and their Primarchs are now daemon princes. The First Founding is basically the family tree for both sides of the Long War.
Key facts at a glance
| Detail | Value |
|---|---|
| Number of Legions | 20 |
| Loyalist Legions | 9 |
| Traitor Legions | 9 |
| Lost Legions | 2 (II and XI) |
| Created by | The Emperor of Mankind |
| Era | Unification Wars / Great Crusade |
| Background book | First Founding, Black Library, 2023 |
Key facts
- The First Founding created the twenty original Space Marine Legions.
- Nine Legions stayed loyal, nine turned traitor, and two were purged from history.
- The loyalist nine became the First Founding Chapters after the Horus Heresy.
- Black Library’s First Founding (2023) covers the nine loyalist Chapters in depth.
- Every later Founding and most successor Chapters trace back to these original gene-lines.
FAQ
How many Legions were in the First Founding?
Twenty. Nine stayed loyal, nine turned traitor during the Horus Heresy, and two (the II and XI Legions) were erased from Imperial records.
What’s the difference between a Legion and a Chapter?
Legions were the huge First Founding formations, tens of thousands of Marines each. After the Heresy, the loyalist Legions were split into smaller Chapters under the Codex Astartes, roughly a thousand Marines apiece.
Are the First Founding Legions still around in 40k?
The nine loyalist Legions survive as Chapters, not Legions. The traitor Legions still exist too, mostly as warbands and Chaos Space Marine forces rather than organised Legions.
Why were two Legions purged?
Nobody knows. The Emperor ordered the II and XI Legions struck from all records, and no official source has ever explained why. It’s one of 40k’s deliberate mysteries.