The Horus Heresy is the galaxy-spanning civil war that tore the Imperium apart in the 31st millennium, roughly 10,000 years before the main Warhammer 40,000 setting. It’s the event that explains why the Imperium looks the way it does today, and it’s the backbone of a huge chunk of the lore.
What actually happened
The Emperor of Mankind was building a human empire across the galaxy, with his 18 genetically engineered sons, the Primarchs, leading legions of superhuman soldiers called the Legiones Astartes. Then Chaos got involved. Through corruption and manipulation, half the Primarchs turned against their father, and the Imperium split into loyalist and traitor factions.
The Warmaster Horus, the Emperor’s most trusted son, led the rebellion. The war that followed was brutal, galaxy-wide, and ended with a confrontation between Horus and the Emperor that left the Emperor mortally wounded and entombed on the Golden Throne. That’s why the Imperium is the decaying, paranoid, Emperor-worshipping place it is in the 41st millennium.
Who’s involved
The conflict is defined by the Primarchs and their legions. Some stayed loyal, like Roboute Guilliman and Sanguinius. Others turned, like Horus, Fulgrim, and Angron. The loyalist-versus-traitor split is the core of the whole story.
It’s worth noting that the exact roster of who fell and why varies across novels and older lore. Different sources emphasise different Primarchs, and some backstories have been retold over the years.
- The Emperor: founder of the Imperium, now stuck on the Golden Throne
- Horus: Warmaster, the Emperor’s most trusted son, and the one who turned
- Loyalist Primarchs: Guilliman, Sanguinius, Dorn, Russ, and others
- Traitor Primarchs: Horus, Fulgrim, Angron, Mortarion, Magnus, and more
- The Legiones Astartes: the superhuman legions that split down the middle
The Black Library novel series
Black Library, Games Workshop’s publishing arm, turned the Heresy into a massive novel series. It started in April 2006 and ran to 65 volumes, closing with The End and the Death, which came out in three parts and finished in January 2024.
The series is written by several authors and has been a big commercial success for Black Library, often hitting bestseller lists. A sequel series, The Scouring, started in December 2025.
The tabletop game
The novels got popular enough that Forge World launched a Horus Heresy tabletop spin-off in 2012, originally as a specialist game with campaign and army books plus resin miniatures. It later grew into a standalone system, and was relaunched as Warhammer: The Horus Heresy in 2022. A tabletop RPG from Cubicle 7 is also in the works.
Why it still matters
Almost everything grim about Warhammer 40k traces back to the Heresy. The Emperor’s entombment, the Imperium’s fear of Chaos, the Space Marine Chapters, the Inquisition, the whole atmosphere of decline and suspicion: it all flows from those years.
It’s also the main reason the setting has such a deep back catalogue. The Heresy gives 40k a foundation myth, and it’s the entry point a lot of fans use to get into the wider universe.
Key facts
- The Horus Heresy is a galaxy-spanning civil war set in the 31st millennium, 10,000 years before Warhammer 40,000.
- It was sparked by Chaos corrupting half of the Emperor’s 18 Primarchs, led by the Warmaster Horus.
- The Emperor was mortally wounded and entombed on the Golden Throne, which is why the Imperium looks the way it does.
- Black Library published a 65-volume novel series from 2006 to 2024, with a sequel series, The Scouring, starting in 2025.
- Forge World launched a tabletop spin-off in 2012, relaunched as Warhammer: The Horus Heresy in 2022.
FAQ
When does the Horus Heresy take place?
In the 31st millennium, about 10,000 years before the main Warhammer 40,000 setting. It’s the civil war that shaped the Imperium into what it is in the 41st millennium.
Who won the Horus Heresy?
The loyalists technically held, but at enormous cost. Horus was killed and the Emperor was left mortally wounded, bound to the Golden Throne, so the Imperium survived in a broken form rather than winning cleanly.
How many books are in the Horus Heresy series?
65 volumes, published by Black Library between 2006 and 2024. The finale, The End and the Death, was split into three parts.
Is the Horus Heresy a game too?
Yes. Forge World launched a tabletop spin-off in 2012, which was relaunched as Warhammer: The Horus Heresy in 2022. A tabletop RPG is also planned.