Ultramarines - Osiris Cluster Rebellion: By the year 833.M30, the XIII Legion had increased in number to an active force of around 33,000 Space Marines, whose primary battlegroup now consisted of the autonomous 12th Expeditionary Fleet. This expansion of their number in a relatively short space of time had been due to two principal factors.The first was that their particular practical style of warfare had a tendency to avoid casualties where possible compared to the tactics employed by certain other Legions. The XIII Legion avoided battles of attrition and prided themselves on achieving strategic goals with the minimum expenditure of life -- and where salvageable Human worlds were involved, this desire was also extended to the minimisation of civilian collateral damage.The second was a latterly revealed aspect of their gene-seed. While the rates of gene-seed implantation success for the XIII Legion's zygote type were very close to the median level, it had proved to be in the highest resistance band to errors during extensive replication -- a meridian matched only by the I Legion's own core sample -- suffering relatively little mutation or deviation in the subsequent generation of harvested Astartes organs. This allowed the Legion to steadily expand its numbers, even without the genetic stabilisation brought by the availability of a primarch's own mature gene-code.At this time, the Emperor, in command of the might of the Principia Imperialis expeditionary fleet, Ferrus Manus and his X Legion and Horus and his Luna Wolves, respectively, led the three main thrusts of the Great Crusade. They were pushing on into the outer void far beyond the Segmentum Solar.But such was the size, self-sufficiency and genetic stability of the XIII Legion that the 12th Expeditionary Fleet alone was entrusted with the task of exploring the extent of the inner galactic disc and its densely packed star systems coreward of Terra. This was why when a secession crisis unexpectedly flared up close to the capitoline systems of the Segmentum Solar itself, the XIII Legion was the closest available Legion and the swiftest to respond.The Osiris Cluster, a grouping of eleven star systems making up the inner portion of the Segmentum Solar's second quadrant, had suddenly and without warning declared secession from the Imperium. Chartist merchant vessels had been seized, Imperialis Armada naval patrol squadrons fired upon and driven off by system defence ships, and agents of Imperial authority rendered silent and assumed dead.The inhabited worlds of the Osiris Cluster, many technologically advanced at contact, had originally come into Compliance during the Great Crusade's eighth standard year in relatively bloodless order. It had been viewed as a highly successful campaign in which the XIII Legion itself had a hand. That the Osiris Cluster had now fallen into open rebellion was deemed an affront to the Legion's honour by its Lord Commander, Gren Vosotho, and the Legion Master of the XIII Legion had vowed to bring the matter to a resolution as swiftly and emphatically as possible.Vosotho, acting on initial intelligence reports, ordered the warships of the 12th Expeditionary Fleet to proceed directly to the halo world of Septus XII, leaving behind the fleet's support elements, lumbering troop transports and forge ships guarded by its slower combat vessels. The target of this rapid strike force was to be the atmospherically-sealed city of Cabasset, located on the night-side of Septus XII, and the economic and political capital of the Osiris Cluster.A strike there before the rebels had time to consolidate their forces, Vosotho reasoned, might end the rebellion in a single, bloody stroke. Breaking into realspace in the outer system, the power of the 12th Expeditionary Fleet's two score of capital warships, led by the Legion's flagship, the Goliath-class macro-battleship Sethaln's Thunder, easily swept aside the system defence monitors and fireships sent out to intercept them. Immediately after the short firefight, in which the Space Marine ships sustained no losses, they engaged in a brief and equally successful long-range precision bombardment to defang Septus XIII's orbital defences, before moving to attack the planet itself.Vosotho had formulated a plan of attack based around a decapitation assault against the governmental and environmental control complexes on which all life in the hive city of Cabasset was dependent for survival. The strategy was a variation of a tried and proven tactic in part developed from close observation of the Luna Wolves' mastery of such attack forms, and which promised a quick resolution with the minimum of damage to the Hive World's infrastructure and an object lesson in the futility of rebellion against the rule of the Emperor.The plan itself was predicated on detailed prior knowledge of the world as an Imperial holding and a military assessment of the potential numbers and capabilities of the rebellious militia forces the Legion was projected to encounter. Unfortunately, every assumption Vosotho and his command staff had made about what awaited the Legion was wrong.