In order to completely inherit the Unique Authority of the 5th Demon King, also known as [Absolute Corruption], Fier trained himself under the tutelage of the Prophet.
Unlike the Demon Kings, who were born with an innate ability to understand and maximize the usages of their own Authority as well as the magic of their "sworn" subordinates, he was merely a lowly demon who possessed an affinity toward this particular Authority of the 5th, or so his savior claimed.
As such, a guide who has a good grasp of how the Authority functions as a whole is needed, and that is the man who wanted him to take up the mantle of a notorious legacy.
Not that he ever objected to it or anything. Rather, it felt like fate had granted him a chance to fulfill his wish.
"Fier, enough of that for today. There's something else I want to talk about."
One day, out of nowhere, his training session lasted only 8 hours compared to the normal 12. Most of it, just like usual, was spent expanding the measly amount of channels within his body as well as improving his control over demonic energy.
However, it seemed his savior/ master had other plans for him that day.
"Have you read the document I gave you the other day?"
He nodded, recalling the piece of paper filled with complex stuff he couldn't wrap his head around.
He did go through the whole thing, though.
"I believe it's time we move on to the next step. So tell me, what have you learned about Ego?"
Ego.
He did see it mentioned several times in the document, but since he didn't understand much of the paper, he didn't know anything at all.
All he could do was show a dumbfounded face.
"Ah... I guess it was a bit hard to grasp that thing right?"
He nodded.
"So I wasn't wrong about that kid."
The man mumbled something and lightly nodded before speaking again.
'That kid?'
"Let's talk about the Soul first." Taking out the same piece of paper, he pointed his hand at what looked to be a puddle.
"This is Soul. Soul, first and foremost, is the core of a being made up of pure Ether energy. It is also a medium that allows one's vessel to interact with the world. When something comes into being, its Soul is the first thing to be born."
Contemplating the man's words, he pointed at the floor, confused.
"That's right. There's a Soul in every "being" there is, but why they don't act like us, I'll get there soon."
Subconsciously, Fier touched his heart, where it usually felt hot whenever he used demonic energy.
"The Soul is unconstrained in its form, so the position it started with isn't exactly fixed. In fact, it is rather evenly spread throughout your body, with only one point of concentration mostly found on the torso."
The man proceeded to turn the page.
"Now, the Soul can be divided into two parts: the Core and the Shell, also known as the Core and the Ego. As it is a medium, the Soul is responsible for assimilating pure Ether energy as well as discharging one's own energy.
Moving on, the Ego is what you would call your "self". It is a manifestation of all the feelings, thoughts, desires, instincts, talents, powers, affinities, ...etc that make up the "you" that is you. This is also what separates us from other "beings".
Conclusively, if the former is the existence of your "being"...then the latter proves that of your "person"."
Fier tilted his head, still somewhat bewildered. He was also curious about the purpose of the lecturing.
"Why am I telling you all this? That's because we're gonna talk about how the 5th's authority made [Corruption] available to just about anyone...and how that is the core of my plan for you."
***
[Corruption] is widely known as a two-step process, where it corrodes the mind and body before the Soul.
In actuality, [Corruption] happens directly to the Soul. Being the egoistic energy that it is, demonic energy boldly invades the Soul and begins the "invasion" upon contact.
Normally, those with inborn immunity and those who with a strong will can easily shake it off without any problem. However, that is not the case with [Absolute Corruption].
This authority allows its owner to directly engage in a fight against the Ego, where the "strength" of two souls is put onto a scale. The weaker soul gets put to slumber, and the stronger rises to take hold of the vessel.
It may sound like a double-edged blade, but with several centuries of arduous Soul training in seclusion under his belt, Fier was confident he wouldn't be beaten in this aspect.
At that point, only the likes of heroes and saints can resist it due to their special relationship with Divinity and Holy Energy.
"While it's indeed true that even [Absolute Corruption] can't ignore a hero's divinity, the hero is by no means immune to it. After all, "Demon King" and "Hero" conceptually counteract each other, so it is only natural their powers are on equal ground compared to others. What you need to do is...simply make them Fall."
One of the first steps to the Prophet's long-term plan is to corrupt the hero. The reason for this is the immediate need to remove the existence of the Hero itself.
For the longest time, "Hero" has always been a concept that brings hope and protection for humanity, which in a way, buffs humanity to an unknown extent against their enemies every time one is born. The same could be said for existence such as "Demon King".
However, if said hero was to fall to the dark side under unnatural circumstances (unrelated to causes such as human nature) and start rebelling against humanity itself, the concept itself would hypothetically crumble, leaving humanity in the same situation as the demonkind in the last 500 years.
Of course, it won't be an instant win for the demonkind but the scale will fall heavily to their side assuming everything goes right.
It was an insane, yet simple plan the madman proposed to the High Council of Dark Lords, of which only three members remained currently to lead the barely surviving 1%.
In any case, if the plan succeeds, the Dark Lords will immediately agree to cooperate with the Prophet, which Fier believes will be the first big win for the demon in such a long time.
At the moment, Fier has already acknowledged the Prophet as the very last hope for his kind, and there's no convincing him otherwise.
"How did I come up with such a plan, you asked?" The man chuckled."You'll see it soon."
Moving on...Surprisingly enough, the plan went smoothly without a hitch, much to Fier's delight.
It appeared that the demon race had become such an insignificant threat in 500 years that...most humans had already forgotten about them.
It was to the point that a majority of them couldn't even figure out what he was despite the obvious aura he purposefully exuded from time to time to play with them.
Of course, everything was moving along nicely...until now...
'Where...?'
On a pasture that seemed to expand infinitely, he, or rather a part of his Ego was seated on a comfy wooden chair.
In front of him was a little boy sitting in a cage, staring at him curiously.