Self-restraint is not one of Beelta's many strengths, but the knowledge that the weird demon might be her only chance for breakthrough made her restrain herself. There is simply too much at stake. She had to take things carefully because she doesn't want to lose her chance.
So eating the weird demon became the very last option. She would try everything she can do to get that access to chaos and if it doesn't work, she will then eat Aeternus.
She decided to question him first and figure out where, how, and why he got the power. But she knew that wasn't going to happen easily. She couldn't walk up to him and ask him about his power and receive answers.
No demon will do that in their right mind. This weird demon is surely not going to spill the beans by being asked politely. The fact that you're asking a question means you need the answer, which means the answer is important to you. Demons will capitalize on that and try to get benefits from you.
Strength can compel a weaker being to talk and torture can force a being in pain to confess but the only way to be sure that a demon is telling the truth is if they are forced to through the power of a contract. If she forms a contract with him then he will be compelled to help her to the best of his abilities. Then she will have the full information to make an informed decision.
To get him to sign a contract, she has to offer him something in exchange. She couldn't use her forces to fight and force him because that might lead to his death. His death might cause the access to chaos to shift to the killer or make it disappear altogether. If he is going to be killed by someone, it had to be her.
So she began watching him. The more she watched, the more she was startled, and maybe impressed. His soul is weak and strong at the same time, probably due to the influence of chaos. His perception is also acute. He had been able to notice her gaze immediately. His intuition is spot on as he can somehow tell which demon is highly dangerous.
The chaos warped his body and also gave him some perks, like making his body unnatural, unfeeling, and untiring. His defenses were boosted by chaos and so is his attack. She couldn't inspect him closely because of the cloak of Chaos energy around him. Chaos energy made his attacks corrosive and it also burned her divine sense.
Those are his strengths. The weird demon may be powerful, but he is not perfect. He has some weaknesses and Beelta found a lot of them. His soul was weak, fragile, and strained. She doesn't know how or why his soul is like that, but she could sense the abnormalities in his soul. His body might be odd but it is weak too, probably the weakest aspect of him. The weird demon always loses clashes that involve physical strength.
The magical prowess of the demon is high but he seems oddly restrained by the chaos energy. Either his soul is too weak to manipulate it or the chaos energy resists his control. She noticed a lag in his spell casting. His sin ability can rectify that weakness but it can only copy the magical attacks of others and has a maximum of three charges. He also cannot fly.
The information about the number of copies he can make came from watching his fights. That information also made her realize that his sin ability might be divine ranked and he might be able to acquire accumulation when next he evolves considering the quality of the chaos energy he is using. But she couldn't let that happen. Accumulation allows a demon to sit back and do nothing while getting stronger. She couldn't let this weird demon have that.
The weird demon is already strong among high-rank demons, almost unbeatable. Accumulation will remove all high-rank demons as a threat to him and she doesn't want him to get into the domain of nobles or they might also sense his unique situation. She has to get him before that happened. So she began using the information she acquired about him.
His movements were erratic, but she wanted to plan an ambush. Then she remembered the time he ran towards the edge of the plane to avoid an ambush. That gave her an idea about how he would behave if overwhelming danger is coming for him. But she couldn't lie about the danger because of his intuition. It had to be real, or he won't be fooled.
She sent out numerous teams to different spots of the border based on his current and estimated locations where they began executing the ambush. The objective of the ambush is to wreak havoc on his body and soul but only to a state where he is close to death and helpless.
The ambush was prepared and she enveloped them in a cloak by using the power of the plane as a demon lord. Her camouflage is not something a demonic beast can see through so Xander didn't see anything when Aeternus sent her to scout.
The demon lord waited for him to get into one of her estimated positions. It was easy to calculate considering that he is looting the fortresses of the demons in the alliance he just fought.
Then she told him about the incoming danger. She didn't lie to him or he might notice something odd. He refused to sign the contract and mocked her before he started to escape. Maybe he wouldn't have mocked her if he knew she was responsible for both the ambush and the attack of the group of high-rank demons.
She may not be able to control the high-rank demons but she can incite them to band together to eliminate a threat to their way of life by using some of her hidden surbodinates.