What Happened at Istvaan III?
The events at Istvaan III marked one of the first open acts of rebellion during the Horus Heresy. Horus ordered loyalist members of his own legions to deploy to the planet and then bombarded them from orbit. This purge eliminated many loyal Space Marines before the wider rebellion became known.
QUICK FACTS
| Item | Detail |
|---|---|
| Event | Istvaan III Atrocity |
| Era | Horus Heresy |
| Leader | Horus Lupercal |
| Method | Virus bombing |
| Outcome | Loyalists purged from traitor legions |
EXPLANATION
After secretly turning against the Emperor, Horus needed to eliminate loyalist elements within the legions that supported him. On the planet Istvaan III, he ordered forces from the Sons of Horus, Death Guard, Emperor’s Children, and World Eaters to deploy to the surface. Once they landed, Horus ordered a virus bombing to destroy them. Some loyalists survived the attack and fought against the traitor forces on the ground before being overwhelmed.
WHY IT MATTERS
The Istvaan III purge revealed the scale of Horus’s rebellion. It removed loyal opposition within traitor legions and allowed Horus to consolidate control of his forces. The event is considered the true beginning of open warfare in the Horus Heresy.
RELATED LORE
What Is the Horus Heresy
What Was the Drop Site Massacre
Who Was Horus Lupercal
What Was the Siege of Terra
FAQ
Why did Horus attack his own troops?
He wanted to remove loyalists within his legions before openly rebelling against the Imperium.
Did anyone survive Istvaan III?
A small number of loyalists survived the initial attack and resisted traitor forces before being defeated.
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