The high-rank demons were fighting all over the place. For someone to control the abyss altar, they need to bind with it. The bonding is very quick if the abyss altar is ownerless. But there will be resistance that will prolong the bonding process if the previous owner hasn't given up control and is still alive.
The resistance can increase the bonding time to a matter of days but it depends on the difference in the spiritual strength of the previous owner and the aspiring owner. That's why anyone that is the current owner will be the target of all attacks. But the fight is more delicate than that.
The fight is all over the place as the owner tries to hide and protect himself. He or she could run away and hide in any part of the domain of high-rank demons but nowhere else. Whoever is bonded with the altar has to be within a certain distance of the altar or they will lose their bond with it. Even with the wide space, the owner can't run away or someone else will try to bind with the abyss altar while he is away. It might take a long time but it is possible.
So that means the current owner must be close to the abyss altar to prevent someone else from taking his right to it. He or she has to weather the attack of others because of this. So he or she must have supporters and allies to protect them and also prevent others from starting the bonding process.
That's why even though the fight is all over the place, it is within the fortress. There may be individual and group fights scattered all over the place but he knows what can get everyone's attention. Few people paid attention to the ominous reaper as he entered but the ones that did scatter like rats before a cat.
"Look it's the ominous reaper."
They began warning each other and soon everyone knew that he was around. They stopped fighting and gathered around to see the freak called the Ominous Reaper. They feared him to be honest. For a demon to fight an alliance and kill the majority of them is a legendary performance. The best that any one of them could do is tie with any one of the demons that he killed. They also have it from a very good source, an eyewitness report, that Aeternus is an abomination. That's way cooler than a legend. But their feelings of fear and respect began to change when they saw where he was heading to.
"Does he want to bind the abyss altar?" Some of them asked each other.
First, there was confusion, followed by indignation, then anger. A mid-rank demon dares to waltz so fearlessly among high-rank demons. Fine, he deserves it. He is strong after all. But to claim the abyss altar right in front of them. That's just insulting. Still, they can stomach it. He is a mid-rank demon and he is already this strong. They could imagine how terrifying he would be when he evolves.
There's a saying in the demon world that the best things in life belong to the strongest. Even with all the logic stacked on Aeternus's side, they felt dejected at the loss, and the thought of leaving filled their minds. There are other available fortresses out there. They would be late to the competition and might be disadvantaged but they wouldn't have to face this terrifying figure.
In short, nothing was worth antagonizing Aeternus. That was before they heard what Aeternus said.
"Yeah, you all better run. You spineless demons. You cowardly dogs should run with your tail between your legs."
"Courting death." One of them shouted indignantly having been angered by what Aeternus said.
Most of them had already begun to turn their backs when they heard what he said. Are they going to leave after he just insulted them? What a story it will be. They can already see it. "The Ominous Reaper strikes again. He claims abyss altar from over 50 high-rank demons and they ran away from his domineering presence after they were scared shitless." it is one thing to hear stories of his feats but it is an awful feeling to be part of his feats especially when they are his opponents.
Some ideas began to creep into their minds. Yes, Aeternus is strong and will likely become terrifying when he evolves. But that "when" is actually "if" or they can make it into "if". There are 51 of them here. Some of them would die in the endeavor but it is more than enough to convert that "when" into "if".
And if they succeed, the story will change to "The Ominous Reaper's reign of terror has finally ended. He bit more than he could chew when he went up against 51 high-rank demons. They put him down after an epic battle and ended his myth of greatness. He wasn't the Ominous Reaper after all, but the Mad Reaper."
There's death and there's glory, high risks, and high rewards. They could feel the promise of greatness coming from their mark of sin. That promise says that if they kill Aeternus and absorb his mark of sin, then they will be great too. But what ultimately pushed them to make the decision is that Wrath is a sin.
They had a demon of wrath among them and this angry demoness whispered to them with her divine sense, "He is courting death. There's 51 of us and 1 of him. If we attack him altogether and at once, we will do him in."
Impeccable logic too. If she had said something stupid it would have made them change their minds. Who would follow the lead of someone with clouded judgment? No one. But she said something that they are all thinking about. Either she isn't being blinded by anger or they are all being blinded by anger. What are the odds of all of them being blinded by anger? "Highly Unlikely."