Other demon kings can create barriers that will be able to match the defense of his body but they will need to actively put up that defense while he doesn't. It means he didn't need to worry about the Celestial of Metal's attacks and the joint attacks of all ten Celestials. He could have just waltzed through them but he hid his strength. He can go all out now that he is covered by the cloud of Chaos energy. No one will see what he is capable of. Even his victims won't be able to see. And if they can, they won't be telling tales after he is done with them.
He practically ripped the three Celestials apart. He dug his hands into their bodies and pulled their Godhoods out from within them while they were still alive. They tried to resist but it was all futile. They were no more than petulant children to him. He ripped out their hearts as easily as taking candy from babies. He watched the life leave them as their bodies fell apart.
He could have killed all five Celestials but he let two go so that his strength will remain hidden. No matter how special his energy is, it will raise eyebrows if he manages to kill all 5 Celestials by himself. He left the other two for his two companions while he kept the 3 Godhoods.
The current Aeternus can be said to be Calamity incarnate. He has grown so much that he can take on the strongest demon kings. The story that other demon kings know for a why such a newly crowned demon king is so powerful so quickly is that Aeternus has a source of Intel that helps him to find vulnerable planes so that his invasions are always successful. They believe he killed a lot of Celestials to acquire the strength he has in such a short time.
It is a plausible explanation for his strength. After all, if a demon king is very lucky, then they can quickly gain strength from invading planes. Invasions usually last tens of thousands of years but good Intel can reduce the time it takes to destroy the defenses of a divine plane. But they are wrong about how strong he is and how he acquired his strength.
He has been a demon for a total of 10,000 years. That is one-tenth of an origin cycle. He grew so strong in such a short amount of time because of Cosmic energy. The activation of Cosmic energy within Helios, when a clone of Legion completed a law, created a lot of changes within Legion. Legion-1's concept of life came into contact with Cosmic energy for the first time and didn't let go.
His concept of life devoured cosmic energy like a tick engorging on blood. Legion-1 practically swelled and was in danger of literally exploding. They tried to save him but nothing could be done to stop his concept from being empowered by cosmic energy.
Legion-1 can control his concept but he can't control cosmic energy. None of them can control cosmic energy except for Legion-9 the tree father, who has a limited manipulation of cosmic energy. However, the tree father can't actively control cosmic energy. He can only coerce cosmic energy within the Authority of the Celestial Supreme at best so he couldn't stop cosmic energy from doing what it wants.
Cosmic energy was activated and it invaded the mind network through the soul sphere. Then it refused to leave. It pumped itself into Legion-1 just like it did with Helios until Legion-1 exploded. Legion-1 did not become a star like Helios. His soul exploded and he died.
His inner world wasn't destroyed by his unusual death. He didn't even stay dead for long. He was resurrected very quickly thanks to the Authority of the Celestial Supreme within his world and the large amount of Cosmic energy that they have access to. It helped his soul to be reconstructed quickly.
His resurrection took less than a month but Legion-1 was not the same when he returned from death.
Legion-1 had become frayed at the edges. The boundaries of his existence had become unfixed. He had imbibed too much Cosmic energy so his existence began to sense the dark side of the universe. His concept has grown past the limit of what an Origin god should have.
All Legion-1 needs is a supreme law and the necessary components to build a world. Then he will be able to safely descend to the dark side of the universe and become a world god.
Other Origin gods need to fuse with more concepts containing cosmic energy and laws with the aid of Origin essence to achieve what he did. It is something that takes thousands of origin cycles. It is already phenomenal that the realm lord could achieve it in 100 Origin cycles. He achieved it in 3 Origin cycles after becoming an Origin god.
But he wasn't exactly thrilled by how it happened. It wasn't a pleasant feeling to be inflated and popped like a balloon. Thankfully his death granted him a reprieve from Cosmic energy. He wasn't force-fed by cosmic energy anymore.
The same thing happened to Legion-7 but he could control his evolution due to his ability to devour laws. The biggest change to him was when he consumed the Supreme law of order of the serpentine world god It evolved to become a massively powerful eldritch being.
All of Legion had one change or the other including Aeternus. Even the unborn clones were affected. As for Aeternus, his Authority of Chaos also engorged on Cosmic energy. He didn't explode from it because his Authority had a clear limit. It can't go beyond level 10 so it had to stop growing.
The only one that wasn't affected by the free and adamant flow of cosmic energy is the tree father. Celestial grade Authority of a Celestial domain can't grow stronger. That is the limit of what it can do.