Magika no Kenshi to Shoukan Maou - 5.1.1 — Strength, the Time It Should be Shown

A skilled but unfortunate modern-day assassin was killed in a freak accident. However, he soon found himself summoned to another world by an evil wizard, who had him possess the body of a boy as an experiment. After successfully planning the evil wizard’s downfall, he used the wizard’s body as a springboard to gain influence and power in his new world.

- Description from Novelupdates

Miguel transmigrated into the Wild Beast World in the body of a useless teen with the same name as himself after he was betrayed by his disciple, the Harem Emperor, for a knowledge and universal secret that he recently uncovered.In the Wild Beast World, humans could contract Wild Beasts through their Summoning Grimoire, a mysterious book that fated individuals can awaken at the age of 18.In this world of summons and beasts, Miguel awakened with the Beauty Sign-In System, which allowed him to contract legendary beauties as long as he signed in at a specific location.On his first sign-in, he received the Holy Daughter of the Red Family, the Fiery Empress of Legend.Using the Wild Beast World and the Sign-in System as a starting point, Miguel swore to return to the Immortal Cultivation World and settle scores with the Harem Emperor once and for all.Follow Miguel's journey as he reaches for the apex, becoming a legendary figure in all worlds.

In novels with a stud there is always a Senior Martial Brother like this:

In the early stages of the novel, his innate talent is high, his cultivation is good, his appearance is handsome, and he has an intelligent, cute younger Martial Sister as a fiancée.

In the middle stages of the novel, he is caught unprepared and defeated by a young disciple (the protagonist) in the sect who’s hardly remarkable at all. From then on, he falls from his godly altar and loses his brilliance. Even his young Martial Sister leaves him for someone else.

In the late stages of the novel, he continues to turn dark, going down the Demonic Path and becoming the major villainous boss. However, the protagonist still ruthlessly defeats him and his soul is eternally sealed.

Once Qin Mo opened his eyes, he discovered that he had become this Senior Martial Brother.

- Description from Novelupdates

Nobody knew at the beginning of “Heaven” that the game itself would be awakened and gain complete self awareness to forid paying for game credits. After 10 years of development, Heaven's game credit has become the only currency interchangeable between reality and virtual reality. Inside the room, a player raises a pistol and points it at his very own head. Previously, he led a comfortable life as the son of a rich couple. But now, he was in a state of misery and destitution because of Heaven. He travels 10 years back in time and returns to the point in time when Heaven was to begin in a week's time. Being reborn, what was Chu You going to decide this time... P. S. The storyline is toxic and the protagonist is hardly sane. He is consumed by vengeance and acts wantonly. For entertainment solely. Read at your own risk.