This prophecy tempted Sigismund later during the battle for the Lion's Gate Spaceport, where he saw Abaddon's banner and had to chose between challenging the Sons of Horus First Captain or instead overseeing a withdrawal of his forces in the face of certain defeat. He met with Euphrati Keeler inside the Imperial Palace, who stated that the Emperor intended for him to slay Abaddon in order to prevent the ultimate victory of Chaos. Дuring the Siege of Terra Sigismund acted as a major loyalist commander but continued to be under Dorn's close scrutiny. Together, they moved to Terra in preparation for Horus' final attack. He led a retreat from Pluto, later reuniting with Dorn as he battled Samus aboard the Phalanx.

Though wounded and forced to teleport away aboard the nearby vessel Persphone, Sigismund was able to cut off Aximand's hand before his departure. During the battle, his Templars fought alongside him against Aximand's warriors, resulting in the death of Lieutenant Boreas. During this second battle at Pluto, Sigismund dueled Horus Aximand as he boarded the Lachrymae. During the final phase of the Solar War shortly before the Siege of Terra, Sigismund commanded against a massive traitor assault from the Frigate Lachrymae. He led a fleet which battled the Alpha Legion during the Battle of Pluto. Later, Sigismund was placed in command of the outermost defenses of the Sol System during the Solar War.

He retained Sigismund as his First Company Captain to avoid demoralizing the men, but disowned Sigismund as a son. Shamed, Sigismund offered his life to Dorn, but the Primarch declined to take it. He showed Sigismund why he had erred out of pride, and that his error was not unlike the pride that drive some of the Primarchs to rebellion. Dorn became enraged, and rebuked Sigismund. Soon after his return to Terra, Sigismund confessed to Dorn his reasons for asking to remain on Terra rather than command the Retribution Fleet.

Upon hearing the news of the Isstvan V Massacre, Dorn ordered Sigismund to purge the Terra solar system of all remnants of the traitor legions. Enemy resistance was too intense to maintain control, and Sigismund's mission was to evacuate as much material and personnel as possible then withdraw from the planet. Sigismund led four companies of Imperial Fists on a mission to secure resources from the forges at Mondus Occulam and Mondus Gamma on Mars. Dorn did not know why Sigismund made this request, but trusted him and did not question it. As a result, Sigismund requested that he be allowed to remain at his Primarch's side, and so command of the Retribution Fleet was given instead to another. His resolve shaken, Sigismund was deeply affected by the conversation. She also told him that his father would need him before the end. One choice would result in his death in space, in the light of an unknown star, forgotten. She gave him a vision of the death of the Imperium, as well as his own. He met Euphrati Keeler who told him Horus would, in fact, go to Terra, and that Sigismund would have to choose where to stand. During the time Dorn spent in isolation Sigismund struggled with the dark news, and felt he lacked purpose. Once Dorn had absorbed the news, he ordered Sigismund as his 'strong, right arm' to go on a mission to the Isstvan System to assist any surviving loyalists and evaluate the situation. It was Sigismund who led the squad of marines that accompanied Rogal Dorn in a boarding action on the Eisenstein, under the command of Battle-Captain Nathaniel Garro of the Death Guard, bringing word of HorusвЂ™s rebellion. Sigismund honors the fallen Boreas during the Solar War

Fists of the warband