The disappearance of one of them will be noticed quickly. They will ask questions and investigate what caused the disappearance of the demon king. This way, he won't be investigated because they believe that they know what he is up to.
They will only say, "He offended the Reaper? That's is entirely his fault. I hope the Reaper will release him soon." And they will move on.
Rumors spread like wildfire in the rumor mill so he decided to take advantage of it instead of failing prey to it.
He kept up this facade by not killing the kings of demons that he captured. Most of the demon kings that he released are kings of demons. Kings of demons have planes. While they keep to themselves mostly and stay within their planes, they also have friends or partners that they work with to invade planes. It will also be quite noticeable that they have died since their plane will disappear when they are killed.
There's no way to hide his murder of kings of demons. So if he wants to kill one, he makes sure to go to their plane and kill them there. He will make a show of it so that no one will suspect him of ulterior motives. No one will question why the plane of a king of demons disappeared and why the demon king itself died. They know that the demon king offended the Reaper and died for it. Such a thing happens a lot. There's nothing much to know or see about it.
All of his acting and lying is stressful but he enjoys it. Sure, he is doing it to keep his nature hidden and himself safe. He learned about mob action when the high-rank demons of the plane he grew up in came together to eliminate him. So it is prudent that he tries not to rile up the other demon kings and demon gods.
But he also does it because he enjoys it and because it enables him to reel in the really big fish. Fear is not a proper tool to catch prey. If the prey is afraid of him, then they will run away from him. Fearless prey are bold enough to offend him thinking that they can get away with it. That gives him the proper cause to retaliate.
Most importantly, when there is a witness, then there is bound to be retribution. This retribution is bound to be big if the witness also knows that he has a lot of wealth. He has found that the best way to lure prey is to dangle irresistible bait at them and also cheat them out of their own share of wealth. He didn't have to wait for long before his bait reeled in a big fish.
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Attacking a demon king's plane is tricky business. First, you have to find their plane in the abyss. It is what a group of 5 demon kings is doing right now. The succubus queen and four other demon kings came together for a single goal. That goal is to assault the plane of a demon king and force him to hand over his wealth. They don't plan to kill him. At least not until he hands over his wealth. To achieve that, they will use words and force when necessary to coerce him.
Again, their main aim is the Wealth of the Reaper. It is what the succubus queen used to tempt these other demon kings to join her. Killing him will not grant them any benefit. Every demon king has a personal space that they created with their Authority. They store their wealth in that space. The space is secure and can only be accessed by the soul of the demon king. So killing the Reaper is not their aim. They will not gain anything that way.
"Does anyone of you have any problems with it?" A canine demon king asked.
To be precise, it is just one of her heads that asked the question. The other two heads are glaring at the other demon kings, daring them to have any objections to what she said. While the head that asked the question is smiling innocently, the ruthless gaze of the other two heads is not encouraging to the demon kings.
This demon king is the leader of this expedition. It is a massive dog-like creature with three heads. She belongs to the race of hellhounds. Her fur is blood-red with sharp rough hair that will tear apart anything that they brush against. It is needless to say that she can't be petted without sufficient courage and protection.
Her fur is the least harmful part of her body. That title goes to her three heads full of sharp silver teeth and a tongue fashioned out of flames. The sharp blade-like claws on her six limbs do not come close to the kind of damage that those silver teeth can do. A single bite can rip the demon kings she is looking at in half.
This hellhound, who is also a demon king of wrath is one of the friends of the snake demon king and also the leader of this group. She wasn't elected into that position through a vote. She is the leader because she is the strongest. The fact that the succubus queen initiated the formation of the group does not grant her the privilege of being the leader. That honor goes to the only level 7 demon king among them.
The other demon kings respect her and have no problem with the hierarchy. They also have no problem with the proposed method that they will use to distribute the Reaper's wealth after they have it. The hellhound demon king proposed that she would take 40% of the wealth while the remaining four of them would take 15% each. Then she asked if they had a problem with it.
One of her heads asked amicably. The other two are glaring at the four of them as if daring them to speak up and give her a reason to use her intimidating teeth on them. It is clear to them what their answer must be.