Warhammer 40k - Ultramarines Short Story - Two Kinds of Fool by Graham McNeill

Captain Cato Sicarius leads his Ultramarines brothers into the space hulk Final Absolution to destroy the alien genestealers lurking within. Vowing to follow the dictates of the Codex Astartes and shed his reputation as a maverick, Sicarius executes his mission perfectly – until the unexpected arises and First Captain Severus Agemman does something unthinkable that may endanger them all.