+++The Horus Heresy+++

The Horus Heresy was a terrible event in the Imperium's History.  The Emperor's most trusted servant, son, and friend, Horus, rebelled against him and took with him nearly half of the Space Marine Chapters and Titan Legions in the galaxy.  This rebellion swept across the Imperium, bringing terror to hundreds of worlds.  Earth itself was attacked, and in a cataclysmic battle above the cradle of Humanity the Emperor defeated and destroyed Horus in personal combat.  The forces of Horus were defeated, but at a terrible cost.  The newly-formed Imperium had almost been shattered, and the Emperor was dying.  The Emperor was kept alive in a Golden Throne, and is still alive to this day, where his psychic presence keeps a watchful eye over his Imperium.