Istvaan III Atrocity

The Battle of Istvaan III, also known as the Istvaan III Atrocity, was the treacherous campaign conducted by the Chaos-corrupted Warmaster Horus that served both to amass those forces loyal to him without suspicion, and also cull those Loyalist elements within their ranks whose loyalties he suspected. The destruction of Istvaan III by virus-bombing and the subsequent internecine battle that followed between the Sons of Horus, Death Guard, Emperor's Children and World Eaters Traitor Legions and their own Loyalist elements was the first overt sign of Horus' turn to Chaos, and the first battle of the Horus Heresy in the early 31st Millennium.