Ultramarines - Great Scouring and the Imperial Reformation: Throughout most of the time that the Horus Heresy was under way, the Ultramarines had been engaged in bringing new worlds into Imperial Compliance on the southern edge of the galaxy. Their very success had carried them far from Terra and isolated them from the conquering Traitor Legions which had been concentrated in the galactic northeast.Consequently, the Ultramarines were poorly placed to contribute to the Loyalist defence during the early stages of the Horus Heresy. Luckily, the Ultramarines were therefore also largely untouched by the fighting of the Warmaster's rebellion, other than the casualties suffered at the Battle of Calth.Other Loyalist Space Marine Legions like the three "Shattered Legions" had lost most of their complement of Legionaries during the fighting, and half of the original 18 Legions had sided with Horus and been corrupted by Chaos. As a result, the number of Astartes left to the Imperium in the wake of the Heresy was very few, yet never had they been more needed by its people.The disorder following the Horus Heresy had left the Imperium weak and extremely vulnerable. Everywhere the enemies of Mankind prepared to attack. Many worlds remained in the grip of the forces of Chaos. Into this breach stepped Roboute Guilliman and the Ultramarines. Always the largest of the Legiones Astartes in the wake of the Rangdan Xenocides, the Ultramarines found themselves divided into smaller units that were despatched all over the Imperium in a desperate effort to stem the tide of invasion and unrest during the campaigns of what became known as the "Great Scouring."The Ultramarines successfully held the Imperium together during this time of intense danger when many centrifugal forces were seeking to rip the edifice of the Imperium asunder. Macragge was able to supply new neophytes for the Legion at such a rate that before long the Ultramarines alone accounted for more than half the total number of Space Marines operating in the post-Heresy Imperium, and there were few Human-settled star systems where their heroism went unnoticed at this time.Within a few solar decades, order had been restored to the Imperium and the demoralised forces of Chaos had been largely driven into the Eye of Terror. Even as the Ultramarines reconquered the worlds lost to the Warmaster and his allies, a new theory of warfare was emerging. Guilliman single-handedly held the Imperium together in the wake of the Horus Heresy, assuming the mantle of the lord commander of the Imperium and Imperial Regent and leading a major reformation of the Imperium's bureaucratic and military structures to ensure that another betrayal from within could never happen again.To prevent a single commander from ever again threatening the Imperium with as many transhuman Astartes at his command as Horus had, Guilliman reformulated the sizes of all of the Loyalist Space Marine Legions into thousand-man "Chapters," breaking apart the 9 original Loyalist First Founding Legions into the much smaller Second Founding Chapters. Never again would one man, no matter how noble his motives, wield the power of an entire Space Marine Legion.Under the guidance of Guilliman's military genius, the Codex Astartes took shape. Its tactical and organisational doctrines would reshape the future of all Space Marines and forevermore dictate the foundation of the Imperium's military strength and therefore the path to survival for Mankind.The majority of the remaining Loyalist Legions were divided into fewer than 5 Successor Chapters so depleted had their numbers been by the civil war, but the Ultramarines were divided many times. The exact number of Successor Chapters created from the Ultramarines at this time is uncertain: the number listed in the oldest copy of the Codex Astartes gives the total as 23, but does not name them.The Second Founding caused dissention among the remaining Loyalist primarchs. Rogal Dorn, Vulkan and Leman Russ opposed the splitting of their Legions while Guilliman's position was supported by Jaghatai Khan and Corvus Corax. Neither side was willing to relent and the controversy grew more intense, with Rogal Dorn calling Guilliman a coward for not having participated in the Siege of Terra, and Guilliman accusing Dorn of being a Traitor and rebel like Horus for his refusal to accept the prescriptions of the Codex.The situation threatened to unleash another civil war upon the Imperium. When the Imperial Fists strike cruiser Terrible Angel was fired upon by the vessels of the newborn Navis Imperialis, Dorn realised that another civil conflict would tear the Emperor's realm apart and finally relented. He agreed to split his Legion.With the Second Founding, the size of the Ultramarines force was much reduced. Most of the Space Marines of the Legion left Macragge to establish new Chapters elsewhere. The Ultramarines' fortress-monastery, the Fortress of Hera, had been built to accommodate more than ten times as many Space Marines as now remained on their homeworld. As a result its arsenals and weapon shops were partially dismantled and taken by the new Chapters to found their own fortresses. The genetic storehouses of the Ultramarines, and the Legion's powerful recruitment mechanism, were similarly divided amongst its successors.Due to this diaspora of Ultramarines successors, the Ultramarines' gene-seed became pre-eminent across the Imperium among the newly-founded Adeptus Astartes. These new Chapters created from the Ultramarines Legion during the Second Founding are often referred to as the "Primogenitors," or "first-born."