The Legiones Astartes, also known as the Space Marine Legions, are the pinnacle of humanity's military might during the Horus Heresy era in the Warhammer 40,000 universe. Created by the Emperor of Mankind, each of the twenty Legions is composed of genetically enhanced super-soldiers known as Space Marines, who possess unparalleled strength, resilience, and combat prowess. These legions are led by the Primarchs, demi-godlike sons of the Emperor, each embodying unique qualities and strategic brilliance. During the Great Crusade, the Legiones Astartes served as the vanguard of the Imperium's expansion, conquering and bringing countless worlds into compliance. However, the Horus Heresy, a galaxy-spanning civil war, saw half of the Legions turn against the Emperor under the leadership of Warmaster Horus. This catastrophic betrayal shattered the unity of the Imperium, pitting brother against brother in a brutal conflict that reshaped the galaxy. The Legiones Astartes, with their advanced power armor, formidable weapons, and unwavering dedication, became both the greatest champions and the most feared adversaries in the tumultuous era of the Horus Heresy.