Chapter Sergeant Sidonus, a veteran Ultramarine, has been through it all. Able to disfuse tense situations with his dry since of humor, he carries an attitude of "been there, done that, no problem." Serving the Imperium for 225 years, he has becom Capt. Titus' closest friend/battle brother. In his long service, he has a bionic arm that replaces the one lost in battle against the Tyranids. The scars on his face he chose to leave to remind him of a victory where an Eldar weapon destroyed his eye and part of his face.
Sergeant Sidonus fights along with Capt. Titus and Leandros as part of the Ultramarine force deployed to combat the Ork invaders on Graia. At one point, Sidonus had to silence Leandros' concerns with Capt. Titus' link to the Warp. Later, Sidonus is charged with taking the mysterious Power Source to their Strike Cruiser where it can be kept safe until it can be handed over to the Chapter Command. It is as he attempts to leave, Nemeroth ambushes and kills Sidonus him by impaling him with his claws.