A "Primarch" refers to one of the twenty genetically-engineered superhuman leaders created by the Emperor of Mankind in the fictional universe of Warhammer 40,000. Each Primarch was designed to embody specific traits and abilities, serving as the head of a Space Marine Legion. They were scattered across the galaxy during their creation, leading to unique cultures and characteristics among them. The Primarchs played a crucial role in the Great Crusade, a campaign to unite humanity under the Emperor's rule. Their legions, composed of Space Marines, fought against various alien races and heretical factions, shaping the future of the Imperium of Man.