The Warmaster was the supreme martial office of the Great Crusade, created by the Emperor when He departed the front to labour in secret upon Terra. Rather than divide command among twenty proud primarchs, He raised one above all others — Horus Lupercal, first-found and best-loved of His sons — and named him Warmaster, first among equals and voice of the Emperor in the field.
The title conferred authority over every Legion, Army, and fleet still on campaign, a burden no being save a primarch could have borne. To some brothers the elevation was simple justice; to others it was a wound, for they judged themselves Horus's equals and resented kneeling to one of their own. That resentment, nurtured in secret, became one of the many cracks through which Chaos poured.
When Horus fell, the office he had been given became the mantle of rebellion. The traitor Legions marched beneath the Warmaster's authority even as it was turned against the Throne, and after his death the name endured as a title of ambition among the champions of the Ruinous Powers. What the Emperor had forged as an instrument of unity had become the emblem of the greatest treason in human history. See also the Horus Heresy explained.