The Battle of Istvaan III, also called the Isstvan III Atrocity, was the first major battle of the Horus Heresy. It’s where Warmaster Horus turned on the Emperor by wiping out the loyalist Space Marines within his own traitor legions, setting the galaxy on fire.
History
The battle took place in 005-006.M31 on the planet Isstvan III. Horus had already been corrupted on Davin and spent over a year secretly preparing his rebellion. He sent loyalist primarchs like Guilliman and Sanguinius far away, then gathered four legions he’d turned, the Sons of Horus, Emperor’s Children, World Eaters, and Death Guard, under the guise of putting down a rebellion on Isstvan III.
In truth, the rebellion was just a pretext. Horus used the campaign to isolate the Space Marines in his ranks who might stay loyal to the Emperor. After the rebels were crushed, he ordered the virus bombing of the planet’s population and then had his loyalist troops purged. This betrayal sparked the Horus Heresy.
Key events
The fighting unfolded in stages. First, the traitor legions fought alongside the loyalists to defeat the Isstvanian rebels. Then, Horus withdrew his loyalist forces to the planet’s surface and unleashed a devastating virus bomb, killing billions. When that didn’t finish off the die-hard loyalists, he sent his traitor legions down to wipe them out in person.
The loyalists made a last stand at the Choral City and other locations. Despite being outnumbered and betrayed, they fought hard, but almost all were killed. A few survivors, like Captain Garviel Loken and Captain Saul Tarvitz, managed to escape or go into hiding. The battle ended in a pyrrhic traitor victory.
- Horus’s forces: Sons of Horus, Emperor’s Children, World Eaters, Death Guard, plus Titan Legions.
- Loyalist forces: Elements of the same legions who refused to turn, about 100,000 Astartes.
- Civilian toll: Over 12 billion Isstvanians died.
- Notable loyalist deaths: Captain Tarik Torgaddon, Captain Saul Tarvitz, Captain Ullis Temeter.
- Notable traitor survivors: Horus, Abaddon, Angron, Mortarion, Fulgrim.
Aftermath and significance
Istvaan III was a disaster for the Imperium. It crippled the loyalist elements within the traitor legions and showed Horus’s willingness to sacrifice anything for power. The battle also served as a rallying cry for the loyalists who survived, and it set the stage for the larger conflict to come.
The Isstvan III Atrocity is remembered as the moment the Horus Heresy truly began. It led directly to the Drop Site Massacre on Isstvan V, where Horus sprung another trap on the loyalist legions. The events on Isstvan III are chronicled in the novel ‘Horus Rising’ and other Horus Heresy books.
Key facts
- The Battle of Istvaan III was the opening battle of the Horus Heresy.
- It took place in 005-006.M31 on the planet Isstvan III.
- Horus used the battle to purge loyalists from his own legions.
- Over 12 billion civilians died in the virus bombing.
- The battle ended in a pyrrhic traitor victory.
FAQ
What was the Battle of Istvaan III?
It was the first major battle of the Horus Heresy, where Warmaster Horus purged loyalist Space Marines from his own legions on the planet Isstvan III.
Why did Horus attack his own troops?
Horus wanted to eliminate any Space Marines who might remain loyal to the Emperor before he openly rebelled. The battle was a trap to wipe them out.
Who won the Battle of Istvaan III?
Horus’s traitor forces won, but at a high cost. Nearly all loyalist Marines were killed, but the betrayal galvanized the remaining loyalists.
What happened after the Battle of Istvaan III?
The Horus Heresy escalated. Horus followed up with the Drop Site Massacre on Isstvan V, destroying more loyalist legions and plunging the galaxy into civil war.