The Sixth is posted in Britannia at some point before the events of Ryse, stationed at York where Commodus resides. Weather or not they campaigned against barbarians before the events of Ryse is unclear.
During the same time frame as the Fourteenth Legion's landing at Dover, King Oswald leads his army with the intent to assault York and Commodus leads the Sixth Legion out to meet Oswald's forces, and the Sixth is effectively destroyed in the ensuing battle, and Commodus is captured.
At least part of the Sixth appears to have survived the battle and returned to Rome or a new force has been raised under the banner of the Sixth, because during Chapter VIII, General Marius Titus orders the Sixth Legion into action during the barbarian assault on Rome.
During the story, it is clearly stated at the end of Chapter III that Commodus leads the Sixth Legion to engage Oswald's army when the Britons march on York, and in the following chapters it is revealed that the Sixth was defeated and Commodus was captured then, but the Chronicle found throughout Chapter IV tells a different story; that Oswald suddenly appears one morning in the forest outside of York, and Commodus eagerly lead just a single cavalry unit in an attempt to capture Oswald and win glory, and is subsequently ambushed and captured. While the Chronicles provide an otherwise consistent backstory, this oversight causes a serious contradiction, as it leaves the fate of the greater Sixth Legion unclear.