Roboute Guilliman - Great Crusade: With its forward base relocated to Macragge, Roboute Guilliman was granted an independent crusade command for the region by the Emperor, and quickly set about a series of fresh conquests. His 12th Expeditionary Fleet reformed under his command and was supplied with warships of the latest designs from Mars as a boon of the Emperor.Fresh conquests were immediate, as the newly named Ultramarines rapidly expanded their range out from Macragge, identifying suitable targets for Imperial Compliance and singling out hostile xenos holds for eradication. Interrupted only when called upon to join larger campaigns by the will of the Emperor, for nearly a Terran century the 12th Expeditionary Fleet ranged as far to the galactic north as the dead expanse where the Dominion of Storms ended and as far to the galactic east and south as the point of Ultima Thule, where the stars paled and emptied out into the limitless darkness of the exo-galactic void.During this period, the Ultramarines, by some records, succeeded in liberating more Human-settled worlds than any other single primarch's forces, and the planets Roboute Guilliman brought within the Imperium always benefitted from his intense passion for efficient and ordered government.Whenever Guilliman and the Ultramarines made a new world Compliant, his forces spent as much effort in establishing it afresh, setting up self-supporting defences, and ensuring that in his wake, the agents of the Imperial Truth and industry would firmly seal the world's place in the fabric of the Imperium. This spread of cohesive Imperial civilisation in the Legion's path served both to solidify and expand supply lines for its advance, facilitating in no small part the great speed and range of the Ultramarines' conquests.As inexorably as its tally of victories multiplied, the Ultramarines Legion's numbers also began to grow. Even before its reunification with its primarch, the XIII Legion had been mindful of its losses and demonstrated its ability to expand steadily thanks to the resilience of its gene-seed during multi-generational, rapid replication. But with its primarch's vision to guide it, his organisational skill and his own genetic material now available to further strengthen and enhance the fidelity and purity of the Legion's gene-stocks, the resultant expansion was meteoric, perhaps beyond anyone's -- even the Emperor's -- expectations.Within solar months of the Legion's establishment on Macragge, the first influx of new recruits had arrived at the Fortress of Hera, the Legion's fortress-monastery and new headquarters, and the process of renewal and increase in the XIII Legion's fighting strength had begun and never since had ceased. Wave after wave of recruits were taken in and processed, not simply from Macragge and the surrounding worlds of the slowly expanding Ultramar, as numerous as they were, but from scores of worlds and colony outposts where the conquering fleets of Roboute Guilliman had gone.The process was meritocratic and exacting; only the strongest and most able candidates were selected, chosen for both desired physical and mental attributes, but the net was flung so wide that the initiate intake dwarfed that of any other single Legion. This, coupled with the Ultramarines' resistance to what they saw as the "waste" of attrition warfare, created a kind of amplification effect in practice where the larger the Legion used in a single Compliance action, the more force was brought to bear and the fewer the casualties taken.Where this was not the case -- and there remained many hard battles fought by the Ultramarines against hideously powerful foes such as savage Ork empires, the Tanaril and other nightmarish xenoforms -- and heavy losses were inevitably suffered, the Legion's sheer size and its huge capacity to renew itself with more recruits offered it formidable resilience, and such losses were almost invariably replaced within a few Terran years.By the time Horus was appointed Warmaster over all the Space Marine Legions and other Imperial military forces at the Triumph of Ullanor, the Ultramarines were by any official assessment the largest single Space Marine Legion by number of Legionaries with a considerable margin. Owing to this expansion, the now massive 12th Expeditionary Fleet was sub-divided into a score of smaller expeditionary and persecution fleets, allowing the Legion to range further, each still numbering scores of vessels and thousands and sometimes tens of thousands of Legionaries.The numeric strength of the Ultramarines Legion, in excess of 250,000 Astartes before the Isstvan III Atrocity, would be an achievement that would not be surpassed, though in secrecy the late expansion of the traitorous Word Bearers, who originally numbered approximately 100,000 Astartes, would come to rival them by some assessments, while the wilder claims as to the strength of the Alpha Legion also have them run closer to the Ultramarines' strength than official records would indicate.This scale of military force and the near-autonomous "empire within an empire" that maintained it -- as Ultramar had reached a size popularly ascribed as the "Five Hundred Worlds" before the outbreak of the galactic civil war -- would have dire and unforeseen consequences for the Ultramarines and their primarch.Separate and inviolate in the east, and a great power within their own right, the Legion's very existence made them a threat to the Traitors' conspiracy that could not be ignored, and on Calth would the Warmaster's plan and the Word Bearers' desire for revenge seek to see that threat destroyed.