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Rogal Dorn expected an honourable battle, but this was not to be. Beginning by isolating the four Companies of the Imperial Fists from their orbital support, Perturabo began to carefully divide his enemy and destroy them piecemeal. Some Imperial Fists managed to penetrate the defences and reach the centre of the Eternal Fortress, only to find there was no central keep – simply an open space watched by yet more defenses. The fortress was a decoy of no real value, surrounded by twenty miles of killing ground. By the sixth day of the siege, Imperial Fists Space Marines were fighting individually, without support, using the bodies of their own battle brothers for cover.{{Cite This}}

The siege of the Eternal Fortress (later referred to as the "[[Iron Cage]] Incident") lasted for a further three weeks. Relief came in the form of Roboute Guilliman and the Ultramarines, who drove off the Iron Warriors, but the siege left Rogal Dorn a broken man and rendered the Imperial Fists Chapter unable to fight for nineteen years. The gene-seed of over 400 Imperial Fists was sacrificed to the Dark Gods, and Perturabo was elevated to the rank of [[Daemon Prince]] of [[Chaos Undivided]] {{Fn|1a}}.