he greatest and fiercest battle for Mankind's survival took place after the Emperor's rise to power in the 31st millennium. Some among the Space Marine Legions, at the urging of Warmaster Horus, turned from the Emperor and embraced the dark gods of Chaos.

Through great treachery, Horus scattered or killed many loyalist Space Marines across the galaxy. Then, with countless thousands of Traitor Marines, the Warmaster laid siege to holy Terra. Brother fought brother outside the Imperial Palace as the streets ran red with the Apocalyptic slaughter. The Emperor ended it by challenging Horus personally, but at great cost to himself.