Demon evolution used to be random before the advent of the system. Evolution will occur once the sin mark upgrades and if there are available resources for it. The system has changed how things used to be done.
In the past, when the evolution process begins, a demon will then upgrade based on the number of resources they have with the priority being in the UPGRADES tab before moving on to the UPGRADABLE.
Only after those two have been sorted out will the matter of the sin ability be determined. By that time, most of the resources if not all of them have been consumed so they mostly get the one at the common rank because it is free. The STATS section will is hardly ever touched.
Nowadays that has changed. Demons get to choose which upgrades they want during their evolution. Still, only the mid-rank demons take advantage of the system. The fledglings and low-rank demons are too stupid to understand the repercussions of their decision, they just want to evolve and grow stronger so that they wouldn't die.
But mid-rank demons are different. They have territories that they farm for resources. When they have accumulated enough, they will then evolve into high-rank demons. It is one of the reasons why high ranks are so much more terrifying than mid-rank demons. The leap in strength from careful planning and wise selection of upgrades is too wide to be bridged with his former strength.
RESOURCES is the tab that lists out the different types of tools that can be used to evolve and in what amount they are possessed. Soul essence is something unique to demons. They are able to digest souls and store them for evolution.
They can also eat the flesh of other beings to get biomass, demon flesh included. The use of divinity is why demons always fight gods. In a way, demons have been engineered to thrive on consuming other beings and grow through killing. Even gods are not exempted from their choice of food.
Aeternus hadn't acquired any soul essence because he used chaos energy to break down everything he kills to fuel the growth of his sin mark. It is a more efficient consumption that doesn't allow for any wastage. It is why he is evolving so soon after being born, well that and the fact that he killed only mid-rank demons. And a lot of them too. He doesn't care about soul essence or biomass anyway. He has Chaos and Originrgy to pay for his needs.
All the options in the UPGRADES tab are a must. They come as a package that will cost 1000 units of any resources. They signify his evolution to the next rank. The UPGRADABLE tab contains the extra options for upgrades. It is not a must for a demon to upgrade all its parameters. If a demon has extra resources after meeting the required amount for evolution, then they can choose from this section.
The STAT section is where filthy rich demons go to splurge. It is the last section in the automatic evolution process, so most demons didn't even know that they could max out their stats during evolution. It was only when the manual evolution option became available that it became known. A certain amount of resources can be used to add 1 to Stats.
But there are STAT limits for each rank. A fledgling starts with Stats between 1 and 9. Each subsequent evolution adds 9 more upgradable Stats to a demon. A normal mid-rank demon should be limited to 27 but Aeternus isn't normal and his chaos energy has refused to follow the trends.
His chaos energy was 54 but it is even forcefully adding an extra +9 stats to itself during this evolution and because his CONSTITUTION, ACTIVITY, MAGICAL RESISTANCE, and PHYSICAL RESISTANCE are affected by his chaos energy, they too get the opportunity to upgrade. But since they are already above normal, they are more expensive to upgrade.
While he doesn't have a choice concerning Chaotic power, he can choose whether to upgrade the others or not. He might stop the others from upgrading but he has to make a CONSTITUTION upgrade or his body won't be able to handle the increased power of Chaos energy. That's also why he must upgrade his SPIRIT.
The options available for upgrade are based on the rank of his mark of sin and should be limited to it but the addition of chaos energy to his existence has broken some rules. His eternal SPIRIT is another outlier due to the quality of his soul. It is the reason why he was able to survive the exposure to chaos energy in the first place.
His soul might be weak and tiny right now but it still has the resistance and perception of an eternal rank soul. It is going to cost more to upgrade it. It's a good thing he has a lot of energy to spare for it. He had been worried about being overwhelmed by the new quality of chaos energy but the upgrade to his soul should keep him afloat.
Last but not least is the sin ability. The sin ability is different for each sin and demons usually get the same one during the evolution to mid-rank. The sin of wrath empowers the holder when they are angry. The sin of pride makes demons faultless and without weakness. The sin of envy longs for the cool abilities of other demons and tries to take them.
Most, if not all low-rank demons chose the free option when they are evolving to the mid-rank. They are ignorant and lack the resources for a better one. But some mid-rank would instead prioritize physical upgrades over the quality of sin ability.
For example, an upgrade of AGILITY vs UNCOMMON sin ability. They cost the same but an UNCOMMON power of envy allows a demon of envy to copy an ability at lower power. It doesn't matter that the demon gets to make two copies since they will never be as good as the original one. It isn't worth it to practical demons.
Meanwhile, AGILITY determines how fast they can run and attack. With a faster AGILITY and higher STRENGTH, demons of envy will be able to make stronger and faster attacks not weaker than that of their opponents. It doesn't take a genius to determine which is better. Which is good because demons are not geniuses.
Aeternus chose the DIVINE quality of sin ability. He had the resources and more to spare. The only resource that he will need in the future is Divinity. Divinity isn't important now but it will become irreplaceable in the future.
The evolution started after making his selection. He didn't lose consciousness but he lost control of his body and he couldn't sense his environment anymore. It was as if he was locked within his mind. Chaos energy worked on his sin mark and his body while origin energy worked in his soul. The evolution took about 30 minutes before it was done.
He hadn't grown an inch in height and his body size hadn't changed at all. His body was still thin and hard. He couldn't even tell his bone from his muscle or skin. He remained the same 1.7 meters he was born with. He was big as a newborn but he will be a dwarf among grown-up demons. Mid-rank demons are well above 2 meters tall while high-rank demons are even bigger.
He sighed. "Chaos energy has stunted my growth."
The haze of chaos fumes around him had grown thicker, his figure couldn't even be seen anymore. What energy that should have made him grow taller had gone into forcefully increasing the Stats of chaos energy.
Only his eyes were visible but they were more like two orbs of dark red flames. The eyes are windows to the soul and it is more so for demons. His soul had grown stronger and it was showing in his eyes. The concept in his soul that he is using to burn chaos energy has become prominent with the development, hence the eye color.
He didn't mind the changes. This is his first evolution so more changes are bound to occur. He was not born a fledgling but a low-rank demon because he wasn't some fragmented mess. But this evolution is making him better and stronger. It doesn't matter how tall he is. If chaos energy is bent on making him stronger, then what's not to like? Might makes right. Height doesn't.
Another thing that had grown stronger is the pressure of Chaos energy on his soul. His soul might have grown stronger but he could feel the distinct possibility that chaos would soon catch up to his resistance and put him back into a soul choke hold. This is highly possible if chaos keeps upgrading itself forcefully everytime he evolves. His battle with Chaos may not be done yet.