A Primarch is one of twenty genetically engineered warlords created by the Emperor of Mankind to serve as demigod commanders of His armies. Each was fashioned from the Emperor's own genetic template, standing far taller and stronger than any ordinary human, gifted with superhuman intellect, charisma, and martial skill. Before they could be raised on Terra, the infant Primarchs were scattered across the galaxy by hostile forces, landing on scattered worlds where they grew to adulthood shaped by wildly different cultures.
During the Great Crusade, the Emperor rediscovered His lost sons one by one, entrusting each with command of a Space Marine Legion bred from his personal gene-seed. The Primarchs led the reconquest of humanity's worlds, becoming legendary figures of loyalty, ambition, and tragedy.
Half of them ultimately turned against the Emperor in the galaxy-spanning civil war known as the Horus Heresy, corrupted by the ruinous powers of the warp. The remainder held to their oaths, though most were slain, lost, or vanished in the centuries that followed. The Primarchs endure as the towering mythic figures around whom much of Imperial history and Space Marine identity revolve.