Cyrus' past is mostly unknown. He's a Scout Sergeant who didn't take to the battlefield in Power Armour like most Space Marines, and what is however known what is known is that his cloak and dagger tactics in battle earned him the attention of the Deathwatch.
In the Deathwatch, it can be reasonable to presume that Cyrus fought many, if not all of the Xeno races that plague the Imperium of Man, including Orks, Eldar and on one occasion Tyranids.
On return to the Blood Ravens, he became a Scout Sergeant again, to which Davian Thule has remarked that within a few centuries there won't be a Blood Raven who hasn't learned from Cyrus' wisdom and knowledge.
During the defence of Calderis, he served on the defence until he joined the Force Commander alongside Avitus. He served as a tactical advisor as well as a squad leader and gave information and details on the Tyranids. Although he, like every Space Marine, claims to "know no fear", he is noticeably wary on the subject of the Tyranids, displaying genuine awe at their terrible power, and expressing a cynical disbelief that anything short of an Exterminatus can stop a Hive Swarm.
In battle, Cyrus is proficient with shotguns, sniper rifles, flamers and bolters.