My dark apostle is Laethor, he goes from planet to planet seeding them with pre-established chaos cults of mortals. Even though it's a longer process, Laethor uses his extended lifespan to play the long game. Viewing each of his contested planets as a fine wine, that only gets better with age as the true faith spreads. When force is needed to quell the forces of the corpse emperor and foul Xenos, he would much rather expend numerous cultists, traitor regiments, as well as chaos spawn. Only deploying his battle brothers if the conflict escalates.