The Cost of Command

The Astral Knights can trace their genetic heritage back to the Imperial Fists. They are stubborn and tenacious, but their highly developed sense of honour can lead to friction between battle-brothers. When a mission goes wrong, two Astral Knights decide the only way to settle the matter is to fight a duel, even though Chapter law frowns on this course of action. Can a fight to the death really resolve the question of who was right, as the victor will be exiled from the Chapter in disgrace?

Read it because
It's two Space Marines coming to blows, and that's not something you see every day. The story contrasts the ebb and flow of their duel with the twists and turns that led to it in a tale that tells you a whole lot about the Astral Knights and their code of honour.