Execution Force - First Execution Force: After learning of Horus' perfidy on Istvaan III, the sires and siresses of the various secret Imperial Assassin Clades, as the temples were originally known during the time of the Great Crusade, were tasked by Malcador the Sigillite, the secret Grand Master of Assassins, with the daunting task of slaying the Arch-traitor Horus.If they could accomplish this monumental undertaking, they could effectively crush the nascent rebellion against the Emperor before it could inflict further damage upon His Imperium.But every assassination attempt against the Renegade Warmaster had failed. Though the operatives sent by the clades were their finest students and equal to the task, every attempt had resulted in failure.The Assassins had thrown their most gifted students into a meat-grinder, sending them in blind and half-prepared.Every strike against Horus was broken, and he had shrugged off each attempt without notice. Every time the Clade masters met, they were forced to grimly listen to a catalogue of each other's failures.After the last failed attempt by Clade Venenum, a new strategy was decided upon. With the advice of a special guest in the form of Constantin Valdor, the Captain-General of the Legio Custodes who served as the personal protectors of the Emperor, the masters of the clades realised that their mission plans were not flawed, they were simply not enough.No single Assassin, no matter how well-trained, no matter which clade they hailed from, could ever hope to terminate the Arch-traitor alone. But a collective of killers, a strike team consisting of an elite unit of Assassins from every clade hand-picked for the task, might be enough to succeed.There had never been a precedent for such an initiative, for the Emperor would have never sanctioned assassination as an official Imperial policy. The deployment of an Assassin was a delicate matter and never one taken lightly.In the past, the clades had fielded two or three of their operatives on a single mission when the circumstances were most extreme, but these Assassins were always drawn from the same clade, and even this occurred only after much deliberation.Yet in this crucial and extreme instance, Malcador, serving as director primus of the clades, decided that more drastic measures were needed to accomplish the Assassins' singular goal. He authorised the creation of the first Imperial Execution Force.Horus did not adhere to the rules of war, nor did he baulk at the use of a tactic because it offended delicate sensibilities. At Istvaan III he had bombed his sworn brethren, his own warriors even, into obliteration. Nothing, no matter how vile, was beyond him.It was decided by the Assassins that if they were to kill this foe, they could not limit themselves to the moral abstracts that had guided the clades in the past. They had to dare to exceed them.And so a select strike team of Assassins was formed. A so-called "Execution Force," the first of its kind. Six Assassins, one from each clade, were gathered together and given the select task of killing Horus by any means necessary.In the meantime, the Dark Apostle Erebus had decided on a bold course of action of his own. He firmly believed that as long as the Traitor Legions followed Horus, all would be as it should and as the Dark Gods had promised.Victory would come soon enough, perhaps even sooner than any of them might expect, for after the latest assassination attempt on the Warmaster, Erebus had come to realise a truism of warfare: if a tactic could be used against Horus, then it could also be used by the Traitors against the Emperor.