From Lexicanum

Sigismund was the first Black Templars Emperor's Champion and originally First Captain of the Imperial Fists. 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. He went forward to challenge the traitorous leaders to single combat, and triumphed over all he met in battle. He was a mighty warrior and after the Horus Heresy and the breakdown of the Imperial Fists into Codex chapters, Sigismund led the more zealous of the Imperial Fists as the Black Templars chapter and was elevated to the rank of First High Marshal of the Black Templars.

Sources

• Codex: Black Templars (4th Edition)