Demon Hierarchy.

Aeternus is far from normal for a demon. He is tireless and he doesn't need to breathe since he doesn't have lungs. Chaos energy has warped his body into something that is an unfeeling vessel much similar to that of an undead. Except he isn't only bones. All of his living flesh has just been morphed into this hard and black material that can conduct Chaos energy.

He didn't encounter any fight all the way here because low-rank demons ran away from him and he didn't bother to give chase. They may have marks of sin because of their evolution but it won't help his own soul mark to grow quickly. 

He doesn't want to spend years trying to become a mid-rank demon by killing low-rank demons. Their power rating which ranged between 20 to 36 didn't encourage him either. They are just too weak for him.