Born to a Cadian mother and a Valhallan Colonel, Jagr was enrolled in the Schola Progenium following the death of his parents in a heretic attack on their command tent. He showed aptitudes in general combat, as well as leadership which made him ideal for the Storm Trooper regiments. His mixed heritage is the reason behind his somewhat confusing name(his last name uses the J as a Y sound), as well as why he was sent from the Storm Troopers to the Kasrkin(the Cadian equivalent).
Jagr served with his squad in many combat zones over a twenty year career, even leading after the death of the original sergeant. However during the Black Crusade, Jagr’s squad finally found a foe they could not beat. While attempting to set up a flanking position to dislodge a stubborn group of cultists, the found themselves face to horrifying face with a Death Guard Champion. Within moments, the squads’ number was halved, las shots either deflected right of the things armour, or made no noticeable impact on the bloated, discolored flesh protruding from cracks in it.