Battle of Isstvan III
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The Battle of Isstvan III was the first battle of the Horus Heresy, carried out by the Traitor Legions to purge any remaining loyalists from their ranks.
As a final preparation for his war on his father, Horus wished to remove all loyalist elements from his and his brothers' forces to prevent them from warning the Emperor of their plans or aiding him in the coming conflict. After gathering their forces in the Isstvan system, the Primarchs, consisting of Horus, Angron, Mortarion, and Fulgrim, ordered all space marines they suspected of being loyalists down to the surface of Isstvan III under the guise of putting down a rebellionnope pretty sure there was an actual rebellion. Once they were there, the Traitors bombarded the planet with virus bombs, catching the loyalists off guard and supposedly fucking the over. While a large number of the loyalists were killed a good number of them
survived and hunkered down, because you cant titrush bombs. Before a second bombardment or Exterminatus could be organized to wipe out the the survivors. Angron led his legion down to the surface to eradicate them personally. Because another bombardment would destroy the Khorne-aligned World Eaters, Horus had a tantrum and the traitors were forced to deploy their own forces. This delay would have extremely large repercussions on course of the Heresy, essentially fucking up Horus' plan from the word go. While the traitors won the battle, it costs them valuable time, during which a small handful of loyalists escaped to warn the Emperor of their treachery, exactly as they had feared. While the traitors won the battle and completed their goal of removing any remaining loyalists from their ranks, the larger purpose of the plan had failed. While this small handful of survivors (most notably Luna Wolves 10th Captain Garviel Loken and Battle-Captain Nathaniel Garro of the Death Guard) wouldn't have a dramatic impact on the civil war like the force of loyalists who were purged would have, they managed to warn the Imperium of Horus's treachery and would become the founding members of the Grey Knights at the order of Malcador the Sigillite aka the Inquisition aka those fucking douchebags.
Tabletop
Forge World's first Horus Heresy rulebook, Betrayal, is focused on this battle; in addition to the generic rules for Space Marine Legions and a Mechanicum detachment, it includes special rules for the specific Legions and characters involved, including Horus, Mortarion,Angron,Abaddon, and Garviel Loken.