Becoming a demon god is pretty straightforward. The most important thing for that to happen is Authority. A demon king accumulates Authority to become stronger and become a demon god. The major reason why Authorities are important to becoming a demon god is so that they will be able to manipulate their mark of sin itself and fuse with it so that they become a sin themselves. After all, a demon god is the personification of a sin. The embodiment of that sin itself.
Every demon gets a mark of sin when they become a low-rank demon. This mark of sin determines when they can evolve and what abilities they get when they evolve. The mark of sin is the root of the power system of demons. It is the foundation of demons and why they use sin energy instead of mana to elevate their state of existence.
