A Warmaster is a supreme military commander, a title granted only in the direst circumstances to place vast Imperial forces under a single guiding will. Because the Imperium learned long ago to fear the concentration of power, the rank is bestowed sparingly, entrusted to a commander deemed capable of winning a war that spans many worlds.
The most infamous Warmaster in history was Horus, the favoured primarch whom the Emperor named commander of all His armies during the great crusade to reclaim the galaxy. It was Horus who then turned traitor, and the catastrophic civil war he ignited bears his name to this day, a betrayal so terrible that the Chaos Space Marines still revere him as the first and greatest of their kind. His treason forever tainted the title with the memory of what unchecked ambition can bring.
In the modern age, the rank endures chiefly in the Astra Militarum, where a Warmaster may be appointed to command the many armies and fleets committed to a great crusade or a war for a whole segmentum. Such a figure wields staggering authority over billions of soldiers, yet always beneath the wary gaze of an Imperium that has never forgotten the lesson of the first Warmaster's fall. It is a title of supreme honour and supreme suspicion in equal measure.