Ryuu Kusari no Ori -Kokoro no Uchi no “Kokoro”- - Chapter 8[LN] Volume 2: .1

Accused by his enemies, Tang Yin was led to a dead end and died at the hands of his enemies. Even so, fate gave him a new chance, and his soul traveled to a lower world, where he managed to reincarnate.His new body had no cultivation and no family either, so he was now a total cripple in an unknown world, with enemies lurking at his door.Yet, he is the proud Divine Heir, dubbed the 'Divine Son' of the Upper Realm. How could something so simple stop him?That is why he will forge his own path and rise again, this time by the hand of a mysterious power that belongs exclusively to him.'Even if I don't understand why I was chosen as Divine Heir, I will create my own path and forge my own power.' He thought.At the same time, a strange and mysterious system with an attractive power, which it said could help him complete his revenge, appeared before him.[You must become the Host of the Domination System][Do you accept?]

Legend says, that there is a great demon, who is the human kind’s public enemy. He is the galaxy federations’ nightmare as he wanted to conquer the whole universe. On the first day, he completely took out the human fleet, plunging them into disorder. On the second day, he declared that he will wipe out the whole federation. On the third day, he declared——that the hero who shot him in their previous battle must marry him, or else, he will destroy everything.

The galaxy federation president and the council members had a long meeting after that. They decided to dispatch a soldier, Wu Xing Yun, and marries him off to the arrogant demon. Wu Xing Yun was once a hero who fought for the nebula, and henceforth, he was tasked—— for the sake human race and the universe’s peace, to eliminate mistresses, settle his matters with the mother-in-law, and fix the man’s crooked ways.

Wu Xing Yun was very perplexed when he asked his commanding officer: “Sir, why did it have to be this way?”

Commanding Officer: “Soldier, bear in mind, that this mission of yours, is for the sake of the universe and mankind’s happiness. You must hold on to your husband, and fight other mistresses! Do you have the confidence to accomplish this task?”

Wu Xing Yun, stood straight, saluted, and instinctively replied: “Sir, yes sir! I will accomplish this task!”

- Description from Novelupdates

There are a total of seven sins, and a revelation is made to a young man named Alex that he is destined to become a demon lord, more especially a demon lord of sin called LUST.

It's not like he can change that and because he always knew something about himself was different—not in a physical sense, well, maybe a little—and not in a mental sense—but rather because he is thrown into a demonic war that compels him to accept his new reality.

He was only aware of a difference in himself. Relating to him and the ladies who are drawn to him... Well, it was more or less like he was a magnet in front of all the girls, and all the girls were drawn to him like a metal.

One day, when a novelist woke up from a restless sleep,he realized that he had transformed into his 12-year-old self.