Fairy Tail: Lost One - Chapter 6: The Hidden Tide

A young man finally managed to become an immortal, a powerful cultivator able to lord over millions of souls. He took advantage of his tremendous luck and fortune to become the youngest immortal in history. He now wants to create his own sect and enjoy life, one filled with female disciples!

Long ago, there were people who stood upon the earth with their heads held high. They never bowed to anyone because of their indomitable spirit. They were capable of controlling wind and lightning, and conquering dragons and serpents. They seemed strong enough to split the earth and shatter the stars with their fists. They traveled throughout the land and called themselves Magi. Eventually, one of them would become a Supreme Magus! These men are the ancestors of human beings. Their blood is what we all share today.

Qing Long is the former strongest man in the world. He traveled through space and time and was reborn as Ji Hao in the Fire Crow Clan of the Southern Wasteland. It’s a complicated world. Forces from both inside and outside of the clan want this young and talented boy to die. Under great pressure, Ji Hao makes a deal with a mysterious man, who resides in his spiritual space, never showing his real face. He gains two drops of blood from a dragon and phoenix. Afterwards, Ji Hao becomes increasingly more powerful.

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

After 2000 years has passed, the ruthless demon lord has just been reincarnated! But his aptitude at an academy for nurturing candidates for demon lords is, “Inept”!?

Having the capability to destroy humans, elementals, and gods, after a long period of countless wars and strife, Arnos the demon lord became sick and tired of all that and longed for a peaceful world, so he decided to reincarnate to the future.

However, what awaited him after his reincarnation is a world too used to peace that his descendants became too weak due to a huge weakening in magical powers.

Arnos enrolled in the Maou Academy established to find any students that might be the demon lord’s reincarnation, but his powers are too extraordinary that those in the academy couldn’t properly judge his powers, so he’s regarded as an “Inept student”. Being underestimated and avoided by most people there, he recruited a girl friendly to him, Misha, as his subordinate and comrade, and strives to climb to the top of the demon race hierarchy to one day reclaim his former title and status.