Shoujo Grand Summoning - Chapter 1577

When an S-class elite assassin-raised captain of a special ops suicide squad comes home from a high-stakes Africa mission, he finds himself offered something that could potentially set him up for life- a curious assignment that he could use to finally exit his days of war and slaughter.

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

It started off peculiarly; first, his domestic cat refused to let him cuddle it.

Lin Qiushi soon found that a sense of disharmony and incongruity began to pervade everything around him.

Then, one odd day, he pushed open a door, and he discovered that the hallway he was familiar with turned into a boundless corridor.

At both ends of this corridor were twelve, identical iron gates.

Thus, the story began.

Ruan Nanzhu said to Lin Qiushi, “When you gaze into the abyss, the abyss also gazes back at you.”

Upon hearing this, Lin Qiushi sank into deep contemplation. He then pulled down the zipper of his trousers and aimed at the abyss…

Ruan Nanzhu: “…Put your pants on properly!”

Super shameless, ill, pampered gong X Shameless, calm shou; a combination of double [or extreme] shamelessness.

- Description from Novelupdates

I am Supreme