Drop Site Massacre - Mobilising for War: Overcome with mind-numbing rage at their brothers' treachery, Ferrus and his warriors gratefully received the Emperor's orders through his brother Rogal Dorn. In response to Horus' betrayal of the Loyalist Astartes in the Sons of Horus, Emperor's Children, World Eaters and Death Guard Legions at Istvaan III, the Primarch of the Imperial Fists Legion, Rogal Dorn, on the direction of the Emperor who had learned of Horus' actions from the Loyalist survivors aboard the Eisenstein. The forces of no fewer than eight Legions had been despatched by Rogal Dorn as acting Magister Militum of the Imperium to crush Horus' rebellion on the world of Istvaan V. They would attack in two waves and fall under the supreme command of the Iron Hands' Primarch Ferrus Manus.The Legions comprising the first wave were the Iron Hands, Salamanders, and the Raven Guard. The Legions comprising the second wave, who would arrive at Istvaan V after the first wave, were the Alpha Legion, Night Lords, Iron Warriors, and a large contingent of Word Bearers that their Primarch Lorgar had stationed in the star system. Also, a large fleet contingent of Dorn's own Imperial Fists were dispatched to investigate Garro's earlier claims. Unknown to Dorn and Ferrus Manus, the Night Lords, Alpha Legion, Iron Warriors and Word Bearers had all turned from their service to the Emperor and pledged their loyalty to Horus, and been instructed to keep their new allegiance to Chaos a secret.Owing to turbulence within the Warp, the Imperial Fists were becalmed and prevented from reaching their goal, but the others made great haste and through a masterful application of Astropathic co-ordination and favourable transit, the avenging forces of the Emperor made swift speed to their target, with the forerunners being the Salamanders and Raven Guard fleets and their attendant forces, with the others following close behind. In the cold light of retrospective vision, this list of seemingly co-incidental facts -- the thwarting of the Imperial Fists fleet, the ease of passage for the others, which Legions should approach the Istvaan System first -- all took on a malign and sinister predestination. In this ordering of events could later be seen the hand of dark forces from beyond, forces which once Mankind slept in blissful ignorance of, but of whose truth the Imperium would later learn at such great cost.It is important to note that at this early stage of the great conflict, the full scale and rank horror of Horus' betrayal were yet to unfold, and both his motivations and the murderous depth of his ambition remained unclear, while the Imperium believed wholeheartedly that it had the upper hand both in matériel strength and capacity to act. There were those among the Loyalists still utterly baffled by what could have triggered Horus' perfidy, and in the private councils of those who knew of the matter -- which was still far from common knowledge -- as the punishment fleet was sent to bring the Traitors to justice, talk of mental breakdown or megalomaniacal insanity on Horus' part, or even the cancerous control of some hideous alien form such as the Khrave or the Enslavers was common speculation as to the cause of this sudden treachery.Regardless of this confusion as to its genesis, the judgement of the Imperium was to be swift and savage in its execution, and Horus' rebellion, swaying as it had four Space Marine Legions to his cause, was judged more dangerous than any that had gone before it during the Great Crusade (and indeed there had been several such), but it was also generally thought that Horus would content himself with carving out an empire of his own as the Emperor's rival. Although clearly a false assumption in hindsight, with this hypothesis in mind, the selection of Istvaan -- a strategically significant star system, seated at the confluence of several known stable Warp routes, but one also far distant from the seat of the Imperium's power -- as a base for this treacherous domain made a certain sense. As did the wisdom of immediate attack before the Traitors' stronghold could be properly established.