The Isstvan System was a remote cluster of worlds that became synonymous with the opening horrors of the war, its planets the stage for the two atrocities that revealed Horus's treachery to the galaxy. Nondescript before the conflict, it would be remembered forever as the place where the Age of Darkness truly began.
Upon Isstvan III, Horus purged his own Legions of those he judged unreliable, ordering a virus-bombing of the planet to exterminate loyal warriors within his ranks. The gambit failed to kill them all, and the survivors waged a doomed ground war amid the ruins, buying with their deaths the first warning that the Warmaster had fallen. Then, upon Isstvan V, came the Drop Site Massacre, where seven Legions sent to bring Horus to justice were betrayed by supposed reinforcements and slaughtered in their thousands.
Together these events made Isstvan the crucible of the civil war. Loyalist strength was gutted, the traitor Legions declared themselves openly, and any hope of a swift end to the rebellion died in the system's ash and blood. To speak of Isstvan in the years that followed was to invoke the moment the galaxy learned that its brightest son had become its greatest enemy.