Horus Heresy - Great Scouring and the Time of Rebirth: As the flames of the civil war subsided across an Imperium in which 2.3 trillion Human beings had died and countless worlds had been utterly shattered, the Ultramarines' Primarch Roboute Guilliman rallied the surviving Loyalists, stretching his own Legion thin across the galaxy to buy time for the other Space Marine Legions to rebuild and rearm to take the fight to the remaining Chaos Space Marine Traitor Legions in what became known as the "Great Scouring" or simply "the Scouring."Although some Traitor Legions had fled to the relative safety of the Eye of Terror, the Iron Warriors continued to hold fast to the worlds they had conquered, and were dislodged only after decades of gruelling warfare. Similarly, the traitorous Night Lords Legion continued their spree of genocide and terror along the Eastern Fringe of the galaxy. This campaign endured until the Night Lords' Primarch Konrad Curze was assassinated on the world of Tsagualsa. Shortly after, Roboute Guilliman believed that he slew the Alpha Legion's Primarch Alpharius on the planet of Eskrador.As the last of the Traitor Legions fled to the safety of the Eye of Terror at the end of the Great Scouring, Roboute Guilliman turned his attention to rebuilding the Imperium of Man itself in what became known to later Imperial historitors as the Time of Rebirth. He took on the supreme military role of lord commander of the Imperium and replaced Malcador as the Imperial Regent, the Imperium's day-to-day civil administrator, with the agreement of his remaining primarch brothers.Guilliman was determined to reshape Imperial institutions so never again would Humanity succumb to a heresy so vast or destructive. Guilliman divided the Imperial Army into the separate branches of the Navis Imperialis ("Imperial Navy") and the Astra Militarum ("Imperial Guard"), diluting their ability to mutiny en masse by creating a bureaucratic rivalry between the services.The  Inquisition had been founded by Malcador the Sigillite at the instigation of the Emperor in the final days of the Heresy to prevent the secret spread of the corruption of Chaos through the ranks of Humanity from happening again, and the Grey Knights Space Marines Chapter was later founded from the core group of 8 psychic Astartes gathered by Nathaniel Garro among his Knights-Errant to combat daemonic incursions as part of what later in the mid-32nd Millennium became the Inquisition's Ordo Malleus.The Council of Terra that had been created by the Emperor after the Ullanor Crusade to provide the Imperium with a more efficient civilian administration was succeeded by the High Lords of Terra: a Senatorum Imperialis composed of the 12 most powerful individuals in the Imperium who were the leaders of its various governmental institutions known collectively as the Adeptus Terra. The High Lords were to act as the regents of the Imperium and rule in the Emperor's name as they sought to carry out His will.Of all the changes brought upon the Imperium by Roboute Guilliman after the Heresy, the most pivotal was the Second Founding of the Space Marines. This reform, enacted seven standard years after the defeat of Horus, saw the remaining Loyalist Space Marine Legions broken up into smaller, more flexible 1000-Astartes units known as Chapters, thus transforming the Legiones Astartes into the Adeptus Astartes.Although Rogal Dorn, Vulkan, and Leman Russ opposed breaking up their beloved Legions, Guilliman's initiative had the backing of Corvus Corax, Jaghatai Khan and the High Lords of Terra, and the measure passed regardless of the other primarchs' opposition, marking the end of the primarchs' dominance over Imperial life and the imposition of mortal rule that the Emperor had desired.Yet, the Adeptus Astartes were granted almost total autonomy from every other Imperial adepta, including the Inquisition, but not from the High Lords of Terra who spoke for the Emperor. Never again would one man wield the power of an entire Space Marine Legion or have such total control over the Imperium.At the same time, Guilliman completed his great work, the Codex Astartes, the famed tome that laid out the proper organisation, tactics and order of battle for a Space Marine Chapter. The Codex soon became a sacred tome for the Ultramarines and their myriad Successor Chapters, the backbone of the Adeptus Astartes in the centuries after the Horus Heresy.Other Chapters, like the Blood Angels, the Dark Angels and the Space Wolves, adopted the Codex's guidelines only in part or not at all, a sign of their continuing disagreement, however ineffective, with the reforms of Guilliman.