Diileem was a regular Space Marine part of the Imperial Fists in the 41st millennium.
He served more than two centuries, bringing victory in the name of his genefather Rogal Dorn, until he was sent on a mission to eliminate a Genestealer cult that was situated on a backwater planet.

Upon his arrival he was met with no welcome, having to fight his way to the Monarch of the cult. He would win, but he was infected with Genestealer DNA, making him morph over the course of multiple weeks. As it got worse, he left the Phalanx star-ship, and continued his duty far from the Imperium's eye, lending his strength in the shadows to other backwater planets.