Well, I decided to go with "save the Emperor" anyway:
A yet greater feat would come to pass with the Feskine Scouring, where the Eagle Warriors fought alongside the Steel Guard and Morning Stars agains the Khrave. Such a concentration of these monsters has never been seen on one planet before or since, and in the midst of the cataclysmic battle, the Emperor and His guards were sundered from the bulk of the Imperial host and surrounded by the malevolent xenos. Faced with utter disaster Orvedui Verigas, Legion Master of the XIIIth, unhesitatingly led his brothers in a valiant charge which claimed a full sixth of the Legion’s warriors, but broke the enemy lines.
By the battle’s end, only three quarters of the Eagle Warriors remained on their feet, but the Khrave were eradicated. On that bloody field, the Emperor declared that the XIIIth would henceforth have the right to bear His personal symbol, the Palatine Aquila, upon their standards and their armour. This constituted an enormous honour and one not dispensed lightly, for the Eagle Warriors were the only Legion to be acclaimed in this manner. From that day on, the XIIIth swore to never falter in their duty and to carry the Aquila to the darkest corners of the galaxy in the newly declared Great Crusade.