His abyssal plane is split into ring-like domains based on the concentration of sin energy. The external ring at the very edge of the plane is the domain of fledglings. A black vine-like plant occupies the areas at the edge of the plane without streams of lava. The streams of lava flow over the edge like a waterfall into somewhere below while the black vine reached up to the edge of the plane from somewhere below.
The black vine is called the demon sire. It is responsible for the production of newborn demon fledglings in all planes of the abyss. The demon sire is said to be an extension of the tree of sin that is situated on the 1st plane of the abyss. The tree of sin reaches down into the abyssal planes like the energy pillar reaches down from the energy sea.
The demon sire is a very prolific plant. It produces large succulent-looking fruits. These fruits are definitely not edible. It is because they are not made for eating. They contain gestating demons.
The large melon-like fruits will burst open when it is ripe and expel the demon fledglings into the plane. The fledglings will fight each and grow stronger. Their need for sin energy will increase as they get stronger so they will move inwards into the plane to get to the places with higher concentrations of sin energy.
Plants form the base of the food chain in a normal plane in the realm but it is sin energy that forms the base of the food chain in an abyssal plane. Demons need more of it in higher concentrations as they get stronger. Without it, they won't be able to regenerate their energy stores and they will stagnate before they are eventually killed off.
The need for sin energy fuels the competition which further leads to competition for space. Space becomes smaller the higher the concentration of sin energy is. The ultimate aim of all demons within the plane is to take control of the center of the plane and become the demon lord.
The center of the plane is where the entity at the top of the food chain resides. Unfortunately for them, this is not a natural plane. There will be no demon lord in this plane. Someone else already owns the entire plane.
At the center of the plane is a large tower. It is a massive black tower that reaches halfway into the sky. That is a height of tens of thousands of kilometers. On top of this tower is a giant sphere of red fire. This sphere looks like a red star and it is ringed with the halo of a golden crown. A large blackish-purple pillar falls down from the sky and enters the star from the top.
The blackish-purple pillar falling from the sky is the energy well of the plane. It passes through the star and continues down into the black tower below. The light of the star provides light for the plane while the energy well provides the sin energy that every demon uses within the plane.
Aeternus is sitting on a throne at the top of the black tower. His throne is a tall and grand structure. It was constructed by using the bones of his enemies to stretch and force the body of another demon king into forming the shape of a throne.
This demon king offended him. It was an offense worthy of death but Aeternus in his infinite kindness didn't kill the demon king. He took some of the demon king's bones out of their body and used the bones to stretch the boneless body into the shape of a throne.
The demon king is very much alive and aware. He is just in so much pain that he can't thank Aeternus for his kindness. It was a great kindness, really. But to be honest, it is also the best decision Aeternus ever made as a demon king. He doesn't regret sparing the life of the demon king because the throne is very comfortable. It is a throne fit for a king.
The throne has great lumber support. He really has to compliment the demon king for being so useful. Unfortunately, the demon king in the throne is much too muddle-headed to appreciate a compliment. Aeternus's words are precious so speaking to the throne will be like wasting pearls on a pig since the throne can't hear him.
The throne is standing close to the energy pillar and beneath the star above his head. The plane sprawled far below him. He can see everything going on within it and he is aware of everything going on in the plane. He is like a god in his divine kingdom.
The plane is his personal domain so nothing here is hidden from him. He can also go anywhere within the plane and do anything he wants. He uses the plane mainly to create soldiers for his armies. He has a structure of incentives to encourage competition within the plane so the demons in his plane are always fighting each other.
He is not paying attention to the killing currently going on in his plane. He sat on this comfortable throne made of a living demon king while he went through his activities in the other planes. This is his body but he has avatars doing his bidding all over the realm. So he may look like he is relaxing and slacking off, but he is bringing death, pain, and destruction to numerous planes right now.
He hasn't changed physically in all these years. He has grown stronger though. It is because of all the successful invasions that he has performed. At least, that's what he tells people when asked about how he managed to grow so strong so fast. He lied but it is their fault if they believe his lie. No one should easily believe the words of a demon.
One of his avatars is currently attacking a very important plane to Legion. It is a plane populated majorly by elementals. They still have their divine plane so he is currently in the Armageddon of the plane waging war on the defenses of the gods.