Surely there has to be one reasonable demon among them that will be the voice of reason. Since no one spoke against the well-thought-out plan to attack the Ominous Reaper, they all attacked.
Every High-rank demon knows a spell or two. They may not specialize in it but since the soul always evolves during each rank-up, their minds have become powerful enough that spells aren't difficult anymore. And spells are the go-to method for quick and ranged retribution against pompous mid-rank demons or any demon of any kind.
Spells flew from all directions toward Aeternus. They hoped to overwhelm him with the sheer amount of them.
Aeternus tsked, "What's with demons and fire spells? It is as if it's the only spell they know."
He clearly isn't being overwhelmed. He had time to complain about how demons only use fire mana. He knew the answer to the question. It's the same reason why the law of fire worked in combination with the law of chaos and why most of the planes of the abyss are volcanic. There are some demons capable of using darkness and spirit but when it comes to bringing out magical firepower, fire is the element that demons are most familiar with.
He has succeeded in goading them into attacking him. It would have been difficult to catch all of them if they had chosen to run away. But they didn't. Instead they came within spell range of him. If their spells can reach him, then the improved version should be more than capable of reaching them.
The coat of darkness that he always wears, morphed and formed various spells that shot out from him. The spells were copies of the ones sent to him, except they were stronger and of a corrosive nature. The spells clashed all around him and created a cloud of chaos.
He poured more chaos into his environment and expanded the cloud.
He chuckled evilly to himself, "Let the hunt begin."
The high-rank demons were still a distance from the point where spells clashed. So when they saw that expanding darkness, they had time to rethink their plan and run. It is public knowledge that you mustn't let the darkness around the Ominous Reaper touch you or you will never see the dim light of the abyss again. But turning your back on a very dangerous enemy is a bad thing to do.
A second volley shot out from the cloud behind them. They hadn't seen it coming at them for most of the way because they were obscured by the cloud. Anything that happens in the cloud stays unknown because of the lack of perception. This time, the demons didn't have a volley of their own prepared. They were trying to outrun their allies now turned to competitors.
If they had prepared their own spells and retaliated, the chaos spells would have been stopped by something, or at least be slowed down which will give them time for better response, like dodging. So the chaos spell fell on them. They weren't so weak as to be one-shotted but most were knocked to the ground while very few evaded the attack. But things didn't end there.
The spreading cloud had become closer to them. They could still make it, but a third volley shot out from it. It was the straw that broke the camel's back. The cloud covered them while they were tussling with the third attack and the rest is easy.
They were right about a lot of things. He isn't a high-rank demon yet and as such as not become terrifying. Anything can still happen to him to prevent him from making that change. They were also right about the cloud of darkness as a trademark skill of the ominous reaper. If you get covered by it, there's no going back.
What they were wrong about is the number of high-rank demons that can turn his "When he becomes a high-rank demon" into an "If he becomes a high-rank demon." 50 average high-rank demons are not enough to make that conversion a possibility.
Four demons survived this encounter with Aeternus. One was very lucky to be at the very back. He had a lot of demons to shield him as he ran. Another one didn't join the crowd in attacking, so Aeternus was a spell short to copy. Chaos energy is unwieldy even for his powerful soul, but it becomes easily malleable when his mark of sin joins in the manipulation through his sin ability. He would be able to create great works with chaos energy then, but outside of it, he is stuck with generic spell structures like chaos bolts and shields. Chaos bolts have the power but not the structural complexity needed to take advantage of that power, so Aeternus let the demon go.
"Trading a fortress for 49 high-rank demons was worth it."
He said as the cloak of darkness receded into his body leaving behind nothing but the eroded ground with groves cut into it in several places by his blades of flaming chaos.
The last two that survived didn't get the memo that the ominous reaper was in town. They were still fighting in a corner unbothered by the trappings of the world. The commotion from the clash finally drew their attention, so they witnessed the demise of 49 demons when they fought the ominous reaper. Word of this feat spread soon after.
When Aeternus reached the next fortress, they were still fighting over the control of the abyss altar, but they scattered and ran as fast as their legs could take them or their flight spells could allow. It didn't matter that they were more than 100 of them, double the amount of what Aeternus killed last time. Could he take on 100 high-rank demons and still come out on top? They didn't want to know and especially didn't want to be part of the experiment to find out.
"There's nothing left for me here anymore, I should move on." He said as he turned towards the direction of another fortress from another alliance.