Brotherhood of the Phoenix - Isstvan III Atrocity: Before Horus openly launched his rebellion to overthrow the Emperor, an opportunity presented itself that would enable him to get rid of the Loyalist elements within the Astartes Legions under his command. The Imperial Planetary Governor of Isstvan III, Vardus Praal, had been corrupted by the Chaos God Slaanesh whose cultists had long been active on the world even before it had been conquered by the Imperium.Praal had declared his independence from the Imperium, and had begun to practice forbidden Slaaneshi sorcery, so the Council of Terra charged the Warmaster Horus with the retaking of that world. This order merely furthered Horus' plan to overthrow the Emperor.Although the four Legions under his direct command -- the Sons of Horus, World Eaters, Death Guard and the Emperor's Children -- had already turned Traitor and pledged themselves to Chaos, there were still some Loyalist elements within each of these Legions that comprised approximately one-third of their forces.The corrupted Fulgrim and his senior officers within the Brotherhood of the Phoenix secretly convened in order to mark those for death that were judged to be fractious or deemed disloyal to their Primarch and the Warmaster.Horus, under the guise of putting down the rebellion against Imperial Compliance on Isstvan III, amassed his troops in the Isstvan System. Horus had a plan by which he would destroy all of the remaining Loyalist elements of the Legions under his command. After a lengthy bombardment of Isstvan III, Horus dispatched all of the known Loyalist Astartes down to the planet, under the pretence of bringing it back into the Imperial fold.At the moment of victory with the capture of the Choral City, the planetary capital of Isstvan III, these Astartes were betrayed when a cascade of terrible Life-Eater virus-bombs fell on the world, launched by the Warmaster's orbiting fleet.Even before the first treacherous blow was struck, Horus' grand plan had, unknown to him, begun to unravel. Before the virus-bombs fell, the impending atrocity had been discovered by several Astartes among the orbiting fleet who remained loyal to their Emperor and their comrades and who resisted.The Loyalist Captain Saul Tarvitz of the Emperor's Children had discovered the far-reaching treachery of the Warmaster and stole a Thunderhawk gunship. He was able to reach the surface of Isstvan III despite pursuit, bringing a timely warning to those Loyalist Space Marines he could find of all four Legions of their impending doom.Those that heard or passed on Tarvitz's warning took shelter before the virus-bombs struck. The surviving Loyalists, under the command of Captains Tarvitz, Garviel Loken and Tarik Torgaddon, another Loyalist member of the Sons of Horus, fought bravely against their own traitorous brethren. The few remaining Loyalists of the Emperor's Children Legion fought bravely on Isstvan III, led by Captains Saul Tarvitz and Solomon Demeter.To prove his worth and loyalty to Lord Commander Eidolon of the Emperor's Children -- and thus to his primarch, Fulgrim -- Captain Lucius of the 13th Company of the Emperor's Children, the future Champion of Slaanesh known as Lucius the Eternal, turned against the Loyalists that he had fought beside because of his prior friendship with Saul Tarvitz. He wanted to punish Tarvitz for taking command of the defence, which had incited Lucius's fierce jealousy of his fellow captain.Lucius slew many of his former comrades personally, an act for which he was then accepted back into the IIIrd Legion on the side of the Traitors. In the end, the Loyalists retreated to their last bastion of defence, only a few hundred of their number remaining. Finally, tired of the conflict, Horus ordered his warriors to withdraw, and then had the remains of the Choral City bombarded into dust for a final time from orbit.Later, at the Drop Site Massacre on Isstvan V, Fulgrim would lead his Astartes in an assault upon the Iron Hands Legion, which was led by the Primarch Ferrus Manus. Ferrus had once been Fulgrim's greatest friend and brother amongst the primarchs, but had rejected Fulgrim's entreaty to join Horus in a betrayal of the Emperor.Fulgrim defeated his brother in battle and beheaded him using the Daemon-possessed Laer blade. But at the moment of Ferrus' death, Fulgrim came back to himself and realised with growing horror what he had done and what atrocities he had been a party to.In that moment of weakness, the Daemon within the blade pounced and convinced the primarch to relinquish control of his own mind. The Daemon then possessed the body of the primarch, holding Fulgrim as a prisoner within his own body.