Sidonus is voiced by actor Richard Hawley.
Sergeant Sidonus, the veteran Ultramarine, had seemingly seen it all. His "been there, done that, no problem" attitude and dry sense of humor instantly served to defuse tense situations. But when faced with the enemy, Sidonus was their worst nightmare – a brutally efficient warrior who wouldn't stop until every enemy lay dead at his feet.
As Captain Titus's closest friend and confidant, Sidonus had served the Imperium for 225 years and beared the marks of this extended battle – including a bionic arm to replace one lost in battle against the Tyranids. When an Eldar weapon destroyed his eye and part of his face, Sidonus decided to leave the scars in place as a reminder of his victory over this formidable enemy.
Players can restore Sidonius Armor in Online using the Relic Armor, Veteran Armor and Battle-Damaged Helmet and arm.
The following text contains plot spoilers.
Deployed in the Ultramarines force along with Titus and Leandros, Sidonus helped to repel the Ork invasion of Graia. However, they unwittingly created a Warp rift by following the directions of Inquisitor Drogan to activate the Psychic Scourge, leading to a Chaos invasion on Graia.
Captain Titus gave the Scourge's mysterious Power Source to Sidonus in order for him to deliver it to their Strike Cruiser for safekeeping until it could be passed to the Chapter command. However, Sidonus was impaled by Nemeroth's claws on a landing pad while attempting to leave the surface, and Nemeroth retrieved the power source.