Horus Heresy · atlas

005.M31 · The Fall · date disputed

Isstvan III Atrocity

Place: Isstvan System · Segmentum Ultima

Fig. 1 · Place on the map

Pyrrhic traitor victory The loyal elements of four Legions are virus-bombarded; but the enemy has lost the element of surprise.

Under the pretext of pacifying a revolt, Horus assembled four Legions at Isstvan III — the Sons of Horus, Emperor’s Children, World Eaters and Death Guard — and with Life-Eater virus bombs exterminated their elements loyal to the Emperor. Billions of Isstvanians perished, but the loyalists, warned by Tarvitz, took shelter underground and waged a fierce ground war for three months.

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Sides

Loyalists

Legions

Commanders

Traitors

Legions

Special forces

Commanders

Type
Massacre
Scale
5 of 5
Casualties
Civilians: 6–12 billion from the Life-Eater and the firestorm; loyalist Astartes ~10,000 killed (inflicting twice as many).

Significance

The first open act of the Heresy; the protracted resistance deprived Horus of a swift secret victory.

Disputed in the canon

  • The virus bombardment began in 005.M31; the ground war stretched into 006.M31 (Fandom marks it “005–006.M31”).

Sources

  1. Lexicanum: Battle of Isstvan III
  2. WH40k Fandom: Isstvan III Atrocity
  3. FW Black Book I: Betrayal
  4. Galaxy in Flames (B. Counter)

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