Istvaan III was a hive world that became the site of the Horus Heresy’s opening atrocity. When Warmaster Horus purged the loyalists from his own Legions there, it marked the first open sign of his fall to Chaos.
What was Istvaan III?
Istvaan III was a hive world in the Isstvan system, part of the Ultima Segmentum. Before the Heresy it held around 12 billion people. It was founded during the Dark Age of Technology and largely escaped the Age of Strife.
The Raven Guard brought it into the Imperium during the Great Crusade, taking it in a single night attack from orbit. Afterwards the Imperium rebuilt it along the lines of the Imperial Truth. By the time of the Heresy it was a dead world.
The rebellion that started it all
The system’s Imperial governor, Baron Vardus Praal, rose in revolt against the Imperium. His rebellion ran for as long as six years before word reached the Council of Terra.
The Emperor, still unaware that Horus had turned, ordered the Warmaster to put down the rebellion. Horus brought four Legions: his own Sons of Horus, the Emperor’s Children, the Death Guard, and the World Eaters. All four Primarchs had already fallen to Chaos, and they used the campaign as cover to purge suspected loyalists from their ranks.
The Istvaan III atrocity
The first wave sent to the surface was made up entirely of suspected loyalists. They deployed by drop pod onto the Choral City, the planet’s capital. Horus cut their communications and held back the usual Stormbirds, leaving them stranded.
Then he virus-bombed the planet. Captain Saul Tarvitz of the Emperor’s Children had stayed with the fleet on a hunch, discovered the plan, and stole a Thunderhawk to warn his brothers. A few hundred loyalists survived in sealed bunkers and shelters. Horus followed the virus bombs with a firestorm that melted the hive cities and razed the world. The psychic death scream of 12 billion people was said to be louder than the Astronomican.
Angron, furious, ignored Horus’s orders and took the World Eaters to the surface to finish the survivors by hand. The battle dragged on far longer than Horus intended.
Key facts
- Istvaan III was a hive world with 12 billion people before the Heresy.
- Horus used the planet to purge loyalists from four Chaos-corrupted Legions.
- The virus bombing and firestorm turned it into a dead world.
- Captain Saul Tarvitz’s warning saved a few hundred loyalists.
- Angron’s disobedience prolonged the fighting on the surface.
FAQ
Why is Istvaan III important?
It’s where Horus first openly turned on the Emperor, virus-bombing his own loyalist Space Marines. That atrocity is considered the first battle of the Horus Heresy.
What happened to Istvaan III?
Horus virus-bombed it and then set off a firestorm that melted the hive cities. The planet became a dead world and never recovered.
Who survived the Istvaan III atrocity?
A few hundred loyalist Space Marines survived in sealed bunkers after Captain Saul Tarvitz warned them. Many were later killed when Angron and the World Eaters landed against Horus’s orders.