I Can Accumulate Proficiency, Starting from Alchemy - Chapter 848 - 364: Eastern Battlefield, Moon Shadow Spirit Camp_1

Leon a young man with the heart of a killer, did something unspeakable before getting transmigrated to another world.And what's more? He found out that due to a wish of his, he got transmigrated to the body of an incubus!And he got into the body at the worst possible time!Something happened, he managed to escape death and became the last of his race...he gained the Vice and Valor System,Lewd and immoral act could give Vice point while battle and bravery can give him Valor Point...This world has magic and beauties that none in his previous life can compare.Join the exciting journey of our MC as he gains immense strength and conquers the most gorgeous girls!

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

The clock struck midnight when Ruelle heard the echo of footsteps. She tensed, the fine hairs on the back of her neck standing on end in the cool night air.“You shouldn't be here,” Ruelle whispered, her voice a breathless murmur. The silhouette finally came to stand under the moonlight, his dark red eyes watching her and his inky black hair ruffling.“Shouldn't I?” His voice was a dark caress, and she stood there captivated by the danger he exuded like perfume.“I haven’t seen you for the last two days,” his tone low. His hand reached out, fingers brushing against the silk of her nightgown, tracing the trembling outline of her collarbone. “Tell me, were you avoiding me, or perhaps... entertaining other offers?”Ruelle’s heart raced, her breaths shallow. She declared, “I don't belong to anyone.”“A bold claim,“ he murmured, his breath a tantalising chill against her skin as he leaned in. ”Yet here you are, pulse racing, your body tensed as if in anticipation of my touch.”His fingers gripped her chin, tilting her face towards his. The moonlight caught his eyes, revealing a glint of predatory intent. “Or must I remind you whose touch you truly crave?”

One will to create oceans. One will to summon the mulberry fields.

One will to slaughter countless devils. One will to eradicate innumerable immortals.

Only my will… is eternal.