MTL - Martial Arts: Just Finished the Patrol, the Beginning is Ruthless - Chapter 38

An 18-year-old poor college student tried to apply for a job as a tutor but his life almost ended as demons' meal. In his last breath, a mysterious woman gave him a strange power. When he opened his eyes he had turned into a demon with a system that would turn him into… A pervert.

Now he must lived with his dual identity as a model student named Ethan and an incubus named Damian.

Childhood sweethearts.
The most popular person in school is my childhood sweetheart.

- Description from Novelupdates

In the legends that were told, ten thousand years ago the twelve demon kings wreaked havoc upon the world, almost completely destroying it. Ten thousand years later, the late arriving Bella discovered that she had been screwed over by her reincarnation, the ranks of the Saviors are already filled and she has to change teams in order to pass the days. Having her gender changed into a beautiful girl… she endured that, not being able to join the righteous side and obtain cheat abilities… she endured, but what she could not endure, was that the world that she had transmigrated to was toxic beyond belief. No one here does their rightful jobs properly, be they heroes, demon kings, or creator gods… in fact, Bella felt as if they all had also started with the wrong faction!

How long will it take for a murdered thief to distill into something monstrous? In this story the darkness only grows stronger, and its reach only grows wider as it expands by devouring the life that surrounds it. This is a dark fantasy where evil is a where, not just a who that focuses on slow progression, undeath, and atmospheric storytelling.