Angron - Istvaan III Atrocity: By the dawn of the 31st Millennium, as the World Eaters' vicious savagery only worsened, many of Angron's brother primarchs voiced their concerns to the Emperor, yet the Master of Mankind, having left the Great Crusade to return to Terra, proceeded to seek help from an unfortunate source. He dispatched Horus, the primarch He trusted over all others, to confront Angron and bring him back into the Imperial fold.Yet Horus was a master manipulator, and unknown to the Emperor, had already himself been corrupted by the Ruinous Powers of Chaos following his campaign to reconquer the Plague Moon of Davin. In Angron, Horus saw a warrior consumed by bitterness and resentment towards the Emperor and it was simple for the Warmaster to feed that bitterness and emphasise the Emperor's betrayal of the World Eater primarch at Nuceria. This fed Angron's perception that the Emperor was a weakling in need of replacement by a stronger ruler -- a ruler like Horus.Horus had told Angron exactly what he wanted to hear. When the Horus Heresy began, plunging the galaxy into civil war, Angron's World Eaters joyfully marched beside the Warmaster into treachery. Thus, the World Eaters became one of Horus' original four Traitor Legions, along with the Death Guard, the Word Bearers and Horus' own Sons of Horus.During the first battle of the Horus Heresy, the Isstvan III Atrocity, Horus at last declared his traitorous hand and openly defied the Emperor. Angron led the World Eaters personally in the first surface assault on Isstvan III to destroy the remaining Loyalist Astartes of the four original Traitor Legions, including his own Loyalist World Eaters, who had survived the traitorous virus-bombing of Istvaan III's planetary capital of the Choral City by Horus' orbiting fleet.Horus had deceitfully launched this treacherous saturation bombardment of the planet after the four Traitor Legions' known Loyalists were already engaged against the Slaaneshi rebels who held the world. The deadly cargo which contained the ferocious, flesh-eating life-eater virus held within the virus bombs killed billions of innocents, whose psychic death scream was said to be louder than the holy beacon of the Astronomican.But much to the Traitors' surprise, nearly two-thirds of the Loyalists from the first wave survived the orbital bombardment, thanks in no small part to the timely warning of the Loyalist Emperor's Children Captain Saul Tarvitz.Taking matters into his own hands, Angron defied Horus' plans and spearheaded a second Drop Pod wave after the bombardment failed to eliminate all of the Loyalists. The Warmaster and his allies could only look on in outrage as the Red Angel made planetfall at the head of a full 50 companies of his bloodthirsty Astartes, landing in the plaza areas to the west of the Precentor's Palace, hunting for their own kin with fratricide in their hearts.The World Eaters bloodily massacred most of their Loyalist Battle-Brothers, plunging into their former comrades' ranks like a white hot dagger. Incensed at his brother's disobedience, the Warmaster saw no choice but to support his ill-tempered and impulsive ally since the atmospheric after effects of the global firestorm unleashed by the virus bombs made it impossible to carry out an immediate, accurate orbital bombardment.Horus ordered all of the Traitor forces to commence a ground attack to salvage victory from disorder. Nearly two full solar months passed on the Dead World of Istvaan III as the Loyalist survivors stalled the Warmaster's plans by tenaciously holding out against the Traitor forces. But their numbers quickly waned against the Traitors reinforcements and steady supply of munitions.Eventually, once the world's atmosphere had cleared enough to make accurate orbital fire once again possible, the Traitors leveraged their superiority of arms, and soon the slaughter swung decisively in the Warmaster's favour following another orbital bombardment of the Loyalist positions. The gauntlet had been thrown down and the Horus Heresy had truly begun.