Sidonus was a grizzled Veteran Sergeant of the Ultramarines 2nd Company who accompanied Captain Titus, and served as his second-in-command. He also served as a mentor to Brother Leandros, a Battle-Brother newly promoted to the ranks of the 2nd Company. Sidonus has served the Imperium of Man for 225 Terran years. A hardened veteran, this hoary old warrior has fought in countless campaigns. He was brutally efficient in combat and would stop at nothing until all enemies were dead at his feet. Despite the grim danger he often faced, Sidonus always managed to maintain his dry, caustic wit to help diffuse the tension of any difficult situation. Sidonus met his fate on the Forge World of Graia at the hands of the Chaos Sorcerer Nemeroth.
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.
Sergeant Sidonus and the rest of his company of Ultramarines were sent to the Adeptus Mechanicus Forge World of Graia in the Segmentum Tempestus to stop an invasion by an Ork WAAAGH! a million of Greenskins strong led by the Warboss Grimskull. Grimskull was intent on stealing advanced Imperial weapons, including a Warlord-class Battle Titan, which were manufactured upon Graia, making the world a startegic asset of the Imperium that had to be retaken. The assault was part of an intricate plan unleashed by the Chaos Sorcerer Nemeroth, the Chaos Lord of a warband of Chaos Space Marines known as the Chosen of Nemeroth, to get his hands on an experimental power source on the world that would allow him to ascend to become a Daemon Prince.
Accompanied by 2 members of his Command Squad, Sergeant Sidonus and Battle-Brother Leandros, Captain Titus joined with the Imperial Guard's 203rd Cadian Shock Troops Regiment and its only surviving officer, 2nd Lieutenant Mira, to eventually kill Grimskull and halt the WAAAGH! Unfortunately, the Ordo Xenos Inquisitor Drogan, who Titus and his Astartes relied upon for assistance in retaking the world, was actually an undead pawn of Nemeroth who used a specially designed weapon known as the Scourge, powered by the experimental power source Nemeroth wanted, to open a portal into the Warp that allowed the Chaos Sorcerer, a horde of Chaos daemons and his warband of Chaos Space Marines to come to Graia.
Curiously, however, Titus displayed an unknown immunity to the sorcerer's psychic powers which immediately raised the suspicions of the highly orthodox Leandros to his commander's possible corruption.
Captain Titus gave the Scourge's mysterious power source to Sergeant Sidonus in order to return it to their Strike Cruiser for safekeeping until it could be passed on to the Ultramarine's Master of the Forge for analysis and if necessary, entombment in the Vaults of Hera with other forbidden and dangerous pieces of technology. Attempting to leave the besieged planet's surface, Sidonus encountered Nemeroth on a landing pad and was impaled by the Chaos Lord's Lightning Claws.
Nemeroth then retrieved the power source and intended to use it to ascend to daemonhood. Following the death of his old friend Sidonus at Nemeroth's hand, Titus hunted down the foul Chaos Sorcerer and slew him just as he was in the throes of the ascent to daemonhood, banishing his dark soul back to the Warp for punishment by his Dark Gods. Unfortunately, after saving Graia, Captain Titus was taken into custody by the Inquisition's Inquisitor Thrax and his retinue of Black Templars Space Marines on suspicion of heresy after Leandros reported his Captain to the Inquisition for both his resistance to Nemeroth's sorcery and his multiple deviations from the Codex Astartes held sacrosanct by all Ultramarines. His current fate remains unknown.