The Legiones Astartes were the transhuman warrior-orders forged by the Emperor to prosecute the Great Crusade and reunite the scattered worlds of humanity beneath a single dominion. Raised from the gene-seed of the twenty primarchs, each Space Marine was a masterwork of martial alchemy, his body remade with implanted organs that granted inhuman resilience, tireless endurance, and the capacity to fight without pause across days of unbroken war.
Organised into twenty Legions, they numbered in their hundreds of thousands at the Crusade's height, their fleets sweeping across the void to compliance-bring world after world. Each Legion inherited the temperament of its primarch, so that the sons of Sons of Horus waged war differently from the sons of the Ultramarines, and no two chapters shared a single soul.
They were the Emperor's finest instrument and, in time, His deadliest wound. When Horus fell to the ruinous powers, the Legions turned upon one another, and the weapon meant to conquer the galaxy nearly unmade the very Imperium it had built.