Yin-Yang Harmony System - Chapter 260

I, the protagonist, was just an ordinary NEET, but suddenly I was reincarnated as a tiny white snake in a fantasy world where legendary creatures and gods exist. Not only that, but I awakened in a cave as the offspring of a terrifying Medusa Serpent.Armed with only my human knowledge and my indomitable mental strength, I'm forced to use cunning tactics and rapidly evolving skills to survive against far stronger monsters and humans... So begins the survival story of a former shut-in with incredible willpower, living as one of the lowest-ranked beasts in a world of magic!ᆖMusic suddenly started playing in my head.I'd been waiting for this moment for so long.'Snakes! Assemble'....

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

Living on the streets of Orchard's Reach, Sen never dreamed about becoming a cultivator. He barely even knew what cultivation was, let alone wanted to do it. When fate intervenes, though, he finds himself the disciple of a cultivator. Like all who find themselves on the path to defying the heavens, Sen will have to decide what kind of person he wants to be and what kind of mark he wants to leave on the world.

~ Erothyria is a world where depravity and fantasy collides ~ And this world is where Dagwood Ackley was thrust into! After picking up the rejected video game called ‘The Daughters of Eros,’ Dagwood is reincarnated into the game and is reborn as a son of Eros! But...it is impossible! No Mother is allowed to begat a son! He takes the identity; Aeric of Starlight, and gains extreme beauty and charm as the first and only ever son of the God Eros. But what is his task? Ding! [Claim all the wives and daughters of Eros] Ding! [Defeat the Goddess of Lust and Claim her!] Ding! [Build a harem of Queen Mothers and daughters and take over the Throne of Eros!] With the Celestial Depravity System, Aeric becomes the most sought after male in all of Erothyria, but he must defend this land from the evil dragons and titans, and in the end, [Claim] it all for himself!