Sergeant Sidonus was Veteran Astartes of the Ultramarines 2nd Company, who had fought across the width and breadth of the galaxy. The numerous scars all over his body were a testament to his long service. This included the loss of one of his arms during an encounter with Tyranids, which was replaced by a bionic one, and the loss of one eye and deep scars on one side of his face after being shot by an Eldar weapon. Having seen it all, he was able to call upon his vast experience to help mentor his junior Battle-Brothers in the arts of war. Despite the ability of the Chapter's Apothecarion to heal such wounds, Sergeant Sidonus elected to leave his scars in place as a reminder of his victories over such formidable enemies.