During Horus's attack on the Imperial Palace, the Imperial Fists' primarch, Rogal Dorn chose his First Captain, Sigismund, as the Emperor's Champion; he was given the best armour and weapon and swore a holy duty to challenge the Champions of Chaos, and triumphed over all he met in battle.

After the Horus Heresy, Roboute Guilliman, the primarch of the Ultramarines, ordered the breakdown of the Space Marine Legions into chapters. Dorn thought Guilliman a coward for not being there during the defence of Terra. Guilliman, in turn, thought Dorn a rebel for refusing to accept the dictates of the new Codex Astartes. Neither would give in. Leman Russ of the Space Wolves and Vulkan of the Salamanders both sided with Dorn, as they did not want their Legions divided, while Corax of the Raven Guard and Jaghatai Khan of the White Scars sided with Guilliman. Again, civil war threatened to split the Imperium.

The Imperial Fists were persecuted for their so-called heresies and even fired upon by the Imperial Navy. When civil war seemed all but inevitable, Dorn finally relented and agreed to split his legion. The first Successor Chapters created from the Imperial Fists were the Crimson Fists and the Black Templars.