Death March kara Hajimaru Isekai Kyusoukyoku (WN) - Book 1 - 1-6. The Marketplace, the Princess and the Slave Trade

Luo Qiu became the boss of a ‘club’ by chance.

It was a weird club that sold strange items and with a servant girl that had 300 years of working experience. Countless people with dreams, hopes and ambitions came to the club to exchange anything precious they own for what they want. They would offer their lifespan, items, and even their soul. Every successful trade would increase Luo Qiu’s lifespan by a little.

“Tribute successful, your lifespan has increased by 99 years.”

As thus, Luo Qiu began his endless life as the club’s boss.

The ordinary world is not ordinary, ordinary people are not ordinary, and safe places are not safe.

This is a story of a passer-by male protagonist who is entangled by various girls with strange hobbies or even non-human beings while surviving in the cracks and opening a harem.

- Description from novelbuddy

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