Sigismund was the first Emperor's Champion (as First Captain of the Imperial Fists). Sigismund had dark blond hair and a patrician face. His build was thick and sturdy. Sigismund was known for being volatile and hot headed, a reputation known widely enough that even Malcador the Sigilite remarked on it in conversation with Rogal Dorn. He was also known for his superior swordsmanship, being regarded as one of the best swordsmen in the Legions.
Sigismund served during the Great Crusade as First Captain of the Imperial Fists and as Adjutant to Rogal Dorn. During the Great Crusade, Sigismund demonstrated a cold, realist view of the future of the Imperium. While conversing with Captains Garviel Loken and Tarik Torgaddon of the Luna Wolves, he stated his belief that the Great Crusade would never end:
"We will spend our lives fighting to secure this Imperium, and then I fear we will spend the rest of our days fighting to keep it intact... In the far future, there will be only war."
He also delivered a striking statement on the nature of the Imperium, perhaps even the nature of man:
"Space is limitless, and so is our appetite to master it."
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 Horus’s rebellion. Once Dorn had absorbed the news, he sent Sigismund as his "strong, right arm" on a mission to the Isstvan System to assist any surviving loyalists and evaluate the situation. Sigismund led four companies of Imperial Fists on a mission to secure the forges at Occulus Mondum and Occulus Gamma on Mars. Enemy resistance proved too intense however, and Sigismund decided instead to evacuate as much material and personnel as possible then withdraw from the planet. During Horus's attack on the Imperial Palace, Rogal Dorn chose Sigismund as the Emperor's Champion, and gave him the best armour and weapons available. He went forward to challenge the traitorous leaders to single combat, and triumphed over all he met in battle. When Rogal Dorn accompanied the Emperor of Mankind and Sanguinius in boarding Horus's flagship, he left most of his Imperial Fists behind to look to the defense of the Imperial Palace. Considering him his finest soldier, Dorn placed Sigismund as the leader of the ground forces until Dorn returned. After Horus had been defeat, but the Imperium's imminent collapse Dorn sent Sigismund along with the other survivors, whilst he and the other loyal Primarchs would start to rebuild here.