A skilled warrior of great renown, Appius took part in the Unification Wars, where the Emperor handed him the laurels of victory at Mesora. He later joined the Great Crusade and took part in the First Pacification of Luna. Afterwards he fought at the side of Horus, when the Primarch was the only one of the Emperor's sons that had been found. Sometime later, Appius joined the Templars and eventually became its Weapons Master. Among Appius' duties, was training those Imperial Fits who had been nominated to join the Company. Sigismund was one of the potential Templars Appius trained and at that time, he was the youngest ever to be considered for the Templars' Temple Oath. During their training together, Appius knew that Sigismund was a great warrior, who would become a Templar and potentially more. After telling the young Imperial Fist this, however, Appius then asked Sigismund what he fought for. All Imperial Fists never took a step backwards, in their fight for the Imperium, but each had their own reason for doing so. For Sigismund, it was to protect those Imperials who were too weak to defend themselves from Mankind's foes. This answer satisfied Appius and Sigismund would go on to join the Templars.
Sometime later, during the Great Crusade though, Appius was mortally wounded in battle and faced dying from his injuries. He, however, refused to accept this and became the first Imperial Fist to chose to be interred within a Dreadnought. Appius survived being interred within a Contemptor and became known as his Legion's Father of Dreadnoughts. He continued to serve with the Templars and later was among the 200 that were chosen to fight Sigismund, as the Templar underwent the trial to become the Company's new leader. Appius was the last to challenge Sigismund, who by then was at his limits. This combined with his Dreadnought's size and strength, nearly allowed Appius to defeat Sigismund. However at the last moment, Sigismund was able to disable Appius' Dreadnought and claimed victory. Their Primarch Rogal Dorn, was there to witness this and proclaimed Sigismund to be the Templars' new Captain.