Maybe a Fake Gintama - Chapter 616: The Moon That the Ancients Saw

To take a dragon's hoard is to challenge death.The greatest of the sea-drakes wakes, curled on his silver throne; but there is no silver. His hoard is stolen. He rises, screaming, and chases the thief to the ends of the world—only to be shot from the sky.But dragons do not die easy, and never one with such festering hatred. A dungeon core rises from his corpse.If he intends to survive so near the cove of pirates that slew his past self, he'll need to master his powers—and quickly. For these pirates are dragonslayers; they won't be felled by mere mushrooms or spiders.Beneath the lawless Calarata, the young dungeon core will have to fight to survive; and one day, he will return death to the man who killed him.

Xin Yun, a sarcastic young man, traveled to an unknown world by chance, but lost his chance to become the strongest because of his obsession with women... or sex. In a collision of the powers of nine powerful men, an incredible thing happened. Xin Yun was reborn back to the time when he traveled to the Isekai world. A fantasy journey is about to begin... [The comic adapted from this novel is popular all over the world]

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

Xu Shulou dreams she’s a character in a bizarre storybook.The male lead is Long Aotian, a man with a “Dragon Overlord” script: treasures fall into his lap, beauties vie for his heart.The first heroine is his delicate junior apprentice sister. The second, the Demon Sect’s saintess. The third… the fourth…Where does Xu Shulou rank? She flips through the plot and realizes—despite being Long Aotian’s official fiancée, she’s nothing but a disposable pawn.In the story, her sect elders dote on her fragile junior sister.“Shulou, yield this treasure to your sister. She’s weaker and needs it more.”“Shulou, don’t be unreasonable. Your gentle sister deserves care.”Even Long Aotian prefers the junior sister. He’s a flirtatious mess, forcing Xu Shulou to compete with other women, her jealousy twisting her into a villain. Meanwhile, the junior sister blossoms into a pristine lotus.But Xu Shulou refuses to play along.When the script demands she and her junior sister be kidnapped, Long Aotian saves the “weaker” girl, saying, “Shulou, you’re stronger—you’ll understand.”In reality? Long Ao Tian arrives just in time to see Xu Shulou slaughter the villains, scoop her sister into her arms, and vanish into the sky—without sparing him a glance.When demons attack, the story expects Long Aotian to leave his fiancée and lovers in the sect’s care. The elders protect only the junior sister, letting Xu Shulou be humiliated.In reality? The elders sneer: “Our senior sister dominates the cultivation world. Why would we need you to worry about her?”When the story’s Long Aotian coaxes the Demon Saintess into betraying her clan for him…In reality, the Saintess timidly asks Xu Shulou: “You’re a righteous heroine. I’m a hunted demon. Why would you want to befriend me?”In the script, Xu Shulou claws at other women in jealous rage.In reality, she’d rather live in peace—until Long Aotian pushes too far. Then she draws her sword.In the story, Long Aotian reigns supreme, surrounded by beauties.In reality, Xu Shulou steals his allies, hoards his treasures, and occasionally beats him up—while he sulks.“My sword saves the world and shields the innocent. It’s not for fighting over men.”“See? Dreams lie. I’m the one holding the Dragon Overlord script.”