The Siege of Terra was the apocalyptic climax of the Horus Heresy, the final and most terrible battle in which the fate of humanity hung in the balance. Having fought his way to the Throneworld, the traitor Warmaster Horus brought his rebel host to bear against the Imperial Palace, the last bastion of the Emperor's dominion.
The defence was a titanic undertaking, marshalling loyalist Space Marines, the golden guardians of the Adeptus Custodes, countless soldiers, and the towering war-engines of the Imperium's forges. Against them the traitors hurled daemon-swollen legions, corrupted Titans, and every horror the powers of Chaos could conjure. The fighting engulfed the entire planet, its skies aflame and its ancient walls broken, as billions perished in a slaughter without equal.
The siege reached its end aboard Horus's flagship, where the Emperor confronted his favoured son in a duel that would decide everything. The Emperor prevailed and destroyed Horus utterly, but was himself mortally wounded, thereafter interred within the life-sustaining machinery of the Golden Throne. The rebellion was broken, yet the cost was beyond reckoning: the dream of a rational, enlightened humanity died on those blood-soaked walls, replaced by the grim and dogmatic Imperium of the ages to come. For a fuller account, see the Siege of Terra.