The submissive and dominate demons.
The submissive demon's purpose is to serve the dominate. To stay with them unless instructed otherwise. If you get marked by a dominate demon, your soul is owned by them.
The dominate demon's purpose is to keep the submissive in check. They can chose to mark any demon they want, and once they do, there is no going back on their choice. They are stronger then submissives and much more powerful.
However. If a dominate demon marks a human or an angel (which is strictly a no go) then they are bound to them. If the human/angel is in a different realm from the demon, it increases the pain as the body tries to remove the mark whilst the demon isn't there.
Tyler, a dominate demon and the son of the devil, hasn't marked anyone. He reckons it's a waste.
So his father, the devil, decides to send him on a rare mission to collect an 'innocent' boy's soul.
Cause the devil loves to be a matchmaker.