The Horus Heresy was a galaxy-wide civil war that raged for nine years, from 005.M31 to 014.M31. It was the most terrible conflict in the history of the Imperium, and it brought the fledgling human empire to the brink of destruction.
The Seeds of Heresy
The Heresy was instigated by the Warmaster Horus, the most beloved of the Emperor's sons. Horus, corrupted by the promises of the Chaos Gods, turned against his father and plunged the galaxy into darkness.
The Civil War
Nine of the twenty Space Marine Legions sided with Horus, while the other nine remained loyal to the Emperor. The resulting conflict was a brutal and bloody affair, with brother fighting brother and worlds burning in the fires of war.
The Siege of Terra
The Heresy culminated in the Siege of Terra, a massive battle for control of the Throneworld. The siege lasted for 55 days, and it saw the death of Primarchs and the destruction of armies. In the end, the loyalists were victorious, but at a terrible cost. The Emperor was mortally wounded, and the Imperium was left scarred and broken.