The Horus heresy

Horus Heresy began when Horus, the Emperor's son, decided he was fit to rule the Imperium, and convinced many others to join him. This led to a civil war that raged for years until the Emperor finally defeated Horus, though both sides suffered heavy casualties. The Horus Heresy was a major turning point in the Imperium's history, as it resulted in the Imperium being split into two parts: the Loyalists and the Traitors.