His entire soul will shatter if he can't withstand the pressure brought about by the addition of the new crown. But he isn't worried at all. His soul has been reinforced by the Authority of a Celestial Supreme. That is why it is in the form of a solid gem and not just a mass of soul power without a definite shape.
The Authority of the Celestial Supreme serves as the anchor of a divine plane. It is strong enough to handle thousands of Celestial Godhoods. Meanwhile, his soul has grown much stronger with the increase in the level of his Authority. The addition of the two makes it so that he is more than capable of handling a hundred crowns.
The glow in his eyes died down after a few moments. It returned to its original state where it is only glowing faintly but it is a little brighter than before. It is almost unnoticeable since his eyes were bright before.
The addition of another crown to his repertoire led to some changes. He became more powerful but that isn't clearly visible. Some other visible changes occurred. A new ring appeared around the star at the center of the plane. It joined the others in circling the star. The other change that occurred was that the throne that he was sitting on groaned slightly. Aeternus has increased in mass so the strain on his poor throne has also increased.
At the point that Aeternus has reached, only a demon king or an equally powerful entity can be used as his throne. He is both heavy physically and spiritually and anything weaker will break if he makes his full weight to settle on it. So he didn't decide to use another demon king just for the fun of it. It is fun to have a demon king as his throne but it is also useful.
He can surely make a seat strong enough to withstand his mass but he prefers this throne so that his visitors will see what he did and believe that he did similar things to the other demon kings that he captured. They will then spread the rumor about his villainy to others. So it is necessary that he sit on a demon king so that others will be fooled about what he actually does with some of his captives.
He is a king of kings so the crowns of other demon kings make him stronger. But he can't go around killing demon kings wantonly or demon kings will hear word of it and decide to gang up on him to eliminate him. So he decided to weave a false story about what he does with his captives to avoid suspicion. It is a story that took a long time to set up. He did it a little by little over time.
First, he captured some demon kings. These demon kings offended him so he was justified in seeking revenge. He tortured them heavily and then let them go. They spread the news of their horrific experience to everyone. What he did isn't unusual. He isn't some psycho that tortures demon kings for pleasure. He did it because of revenge. So no one thought much of it or suspected anything.
Then he captured more demon kings. Some offended him while most didn't. He released the few that offended him so that they could tell others about the unfortunate thing that happened to them.
As for the innocent ones, he took their crowns and killed them permanently. He made sure never to kill them in front of anyone. Those that he kills never return and nothing is heard about them. They will never be able to tell anyone that they were captured wrongfully.
What the other demon kings will hear is that he captured some more demon kings that offended him. He tortured them, and eventually released them. After all, they have witnesses to the event. That's why he will ask two demon kings to accompany him in invading a plane when he can do it himself. It is so that he can capture one and release the other as a witness.
It is important that there is a witness to see his act of capturing demon kings. It is so that other demon kings will know what he did with the captives. They will assume he captured them for a power fetish. A witness will also have detailed information about his strength and why he captured the demon kings. It is important that they know because people, be they demons or normal races, do not fear what they think they know.
They will know that he is not an unreasonable maniac. All the captives that he captured did something to offend him. They will know that he can be worked with as long as they don't do something to piss him off. He will be able to ask other demon kings for collaboration that way. They won't have any reason to fear him so they will continue to accept his offer for collaboration. Some will even still look down on him and underestimate him because they think they know his strength.
If he did everything secretly, then assumptions about him would go wide. No one will know how strong he is and why he captured the demon kings. They will fear him because of that. No one will want to associate with him.
It is very important that they think that they know what he does with the captives. It is so that they will not suspect him of being a king of kings. After all, demon kings gain nothing by killing each other. And that's also why he must not kill and take the crown of another king in any place outside the privacy of his plane. It is practically announcing that he is an outlier.
All his patient machinations and meticulous scheming are done because most demon kings know each other. Kings of the abyss don't have a plane to themselves so they all stay on the 1st plane of the abyss. They communicate with each other and share rumors.