A high-rank demon can not defeat a demon noble, and neither can a new demon noble defeat an old one. This is because of a certain unique ability that noble demons have called ACCUMULATION. It is the ability to store some of your power away, a little at a time, for future use.
It is this ability that makes Salazar certain that he would be able to beat the ominous reaper no matter how special he is. A demon noble gets stronger the longer they have been a demon noble. The little power that they save everyday easily becomes a ridiculous amount when the years go by. The Ominous Reaper is a baby and will never be able to stand up to his ten thousand years of Accumulation as a demon noble.
A low-rank demon acquires the mark of sin and gets access to sin energy. A mid-rank demon evolves the mark of sin and gets an active sin ability. A high-rank demon gets another sin ability but a passive one this time around. But when noble demons evolve due to successfully fusing mana and sin energy, their active and passive abilities fuse to create a passive accumulation of power that can be actively used. This is the norm.
But there are some special demons that break the norm. They were able to acquire a divine rank sin ability when they became mid-rank demons and their comprehensive prowess is high enough to fuse mana and sin energy as mid-rank demons instead of as high-rank demons. When those demons evolve to the high-rank stage, they get the opportunity to choose two sin abilities instead of one or they can pick ACCUMULATION.
Bael is one such demon. To achieve this unique ability, a lot of resources are needed but not only that. They also have to be geniuses. To create such an entity, Salazar had a lot of kids, fed them, and pit them against each other in regular battles to death. The winner will eat the loser and grow stronger so that all the resources that went into raising the loser won't go to waste.
Salazar also limited their diet to each other since they have his genes. He didn't want them to be contaminated with another demon's genes too much even through their food. If Bael wasn't strong, he wouldn't have survived his childhood.
A demon noble can provide enough resources to allow numerous low-rank demons to evolve into the mid-rank with a divine sin ability but the genius and ingenuity needed to fuse sin energy and mana at such a level needs conflict, danger, and of course genius. Normal low-rank demons are still brawling on the plane while mid-rank demons are fighting over demonic beasts and territories. Something like spell casting is impossible for most mid-rank demons much less mastering sin energy and mana. That just shows how difficult the endeavor is.
Succeeding in it might not be a matter of resources but Salazar sunk a fortune into the endeavor and of course, a lot of his children. It might be controversial but it is normal in the demon society. Demons kill and eat each other every time, why can't a demon father force his kids to kill and eat each other? In fact, he doesn't need to, it happens every time too. Demon siblings kill and eat each other.
No matter how controversial or frowned upon it is, Salazar succeeded in creating Bael. It is rare, but his genes were that good. Bael is unrivaled amongst high-rank demons because of his centuries of accumulating power. He can even kill newly evolved demon nobles and some weak demon nobles. Imagine how strong he will be when he becomes a demon noble.
It is why Bael is confident in himself and is unfazed about Aeternus's might. It is also why Salazar will not risk Bael. As a demon noble, Salazar understood more than anyone else that "when" can become "if." Bael isn't a demon noble yet, anything can change to make that future disappear with his hopes and dreams of becoming the next demon lord.
Back to Aeternus.
He had been running for days and yet, nothing happened. No one attacked him and everything was calm. But he wasn't feeling at ease. He could tell that something bad was about to happen the more time passed by. It was like he was being chased and the distance between him and danger was increasing rapidly. He picked up speed but the sensation continued to increase. He could tell for a fact that he was in danger and the demon lord wasn't lying.
"Just have to cross the border. Everything will be okay then."
That's what he kept telling himself as the days went by. He ran for five days straight with that constant feeling of dread creeping up on him. His non-existent heart was already in his throat by the time he reached the border. The danger was close to him now, very close. But he couldn't see the danger.
"It doesn't matter. It's too late for them now. I'm almost there." He said to calm himself down.
That's when it all happened. A force as heavy as a hammer struck his head and went through it. The force targeted his soul in a piercing manner as if it would destroy it once they made contact. They made contact and the force shattered harmlessly against his soul. He didn't even feel a twinge but he knew he was under attack and the attacks weren't over.
"Could they have finally caught up to me?" He thought as more powerful soul attacks struck him.
Then he looked up when he sensed the new threat. Some boulders had somehow made their way into the air above him and were falling down now. A demon appeared amongst them and began to light them on fire. The boulders became flaming boulders. It was then that everything came into view.
Aeternus realized immediately. "They were waiting for me."