Imposters are killed. It is a reality that has been proven in the abyss. You will lose your life if you claim to be something and you don't have the strength to back it up. If you claim that a fortress in the abyss is yours and that you're in control of its abyss altar, your claim will be tested. It doesn't matter if you're truly in control of the abyss altar. What matters is if you can defend your claim to it. Failure to do anything in the abyss almost always leads to death.
The will of the abyss became silent with that warning. His surroundings suddenly lit up and changed. He flinched first before he figured out where he is. He has appeared beside a demon. This demon is in the domain of fledglings at the edge of the plane because the other demons around are small weak things. This demon is strange. It is bigger than fledglings, about the size of a low-rank demon and black smoke is oozing out of the demon's body. The other fledglings give it a wide berth.
'This is me.' He realized.
The will of the abyss spoke again. "A king should have fortitude."
The surroundings warped as time flowed faster. It showed Aeternus standing up after 50 years then fighting his first demon, a mid-rank demon.
'Fitalo the demon of wrath.' he muttered to himself.
He felt pain in his eyes as his former self poked Fitalo's eyes, then he felt phantom pain between his legs as his former self struck Fitalo between the legs. Fitalo swore to kill him painfully. Next came the pain that Fitalo felt as he was burned to death by Chaos flames.
"This is nothing to me." He said confidently.
He felt pain for everything he inflicted on Fitalo but the pain felt less than an itch. The pain of the eye pokes was more than the pain of the death by Chaos flames because he is more acclimated to chaos than pain. Even then, the pain is almost nothing. So he is confident that he will survive this trial if this is all that it will take to become a king.
His confidence didn't change as the record of his life as a demon played. He felt every pain he ever inflicted on others. Most of them were by chaos flames so it was a breeze. But there are certain occasions when he cut someone's head off, cut their arms, or disemboweled them. Those came back to bite him in more ways than he thought.
His mind took on the pain easily so he thought that the trial is easy, but it is not so. Because in the real world in the Zargoth plane, his body isn't having it so well. The eye sockets of his body were poked when he felt the pain of being poked in the trial world. Since he doesn't have eyes, there was no damage to his body. The same thing happened when he felt kicked between the legs. He has nothing there to be damaged so he was not damaged.
The Chaos flames did no damage to him at all either. It is the other kinds of injury that he caused to others that are damaging his body. His head and arms were cut off. His torso was mutilated and crushed. His legs were broken and his spine shattered. Every form of physical damage he caused to others began to reflect on his body.
The trial of the abyss tests the mind and the body individually. The mind and body will be tortured and since they are made differently, they react differently to the same thing. His mind can feel pain but its threshold for pain is very high. His body has been warped by Chaos energy and can't feel pain at all which is a good thing normally, but it also can't heal without Chaos energy.
In a normal situation, damage to his body will heal quickly and he can heal almost any amount of damage because of his infinite supply of energy but this is not a normal situation. His mark of sin and his soul are currently in limbo until he passes his trial of the abyss. That means his body can't get the Chaos energy it needs to heal.
His body is an anchor that tethers Aeternus to the world of the living. If his body doesn't heal, it will become damaged enough that the link between Aeternus's soul to the world is destroyed. His soul will remain stuck in limbo and will never return. He will die and be forgotten as one of the failures of the trial of the abyss.
This is the most common cause of death in other demon lords taking the trial of the abyss. Their fleshy bodies experience all the damage that they have ever inflicted. It is an unreasonable thing to ask for considering the lifestyle of demons. Except their body can heal on its own and they can use life essence to keep their body together. Aeternus's body is not natural, life essence doesn't work on it. Divine life energy might work but Legion is not willing to take the risk of allowing Chaos energy to infiltrate the mind network. So Aeternus's body is on its own.
Unlike Aeternus, other demon lords are more threatened by the pain in their souls than the damage to their bodies. Soul damage is very difficult to heal and life essence can't heal it because the soul has been pulled away from the body. But as long as they can withstand the pain in their minds, they will survive the trial of the abyss if they have life essence for their bodies.
Aeternus has always been an odd demon, an outlier. The threat of this trial is to his body, not his mind. He continued to feel carefree while his body got destroyed bit by bit. Each damage to his body reduced the connection of his soul to his body. He lost his anchor to the world bit by bit as the trial continued. He remained clueless about his precarious situation throughout the process because he can't feel pain.