MTL - Song in the Peach Blossoms - Chapter 1 The story is like this

A man who was once called the world’s strongest agent ended up becoming a teacher after his retirement, to train a new generation of agents. After many years of training his disciples, he was killed at the age of 60 by a secret organization’s ruse and was reincarnated in another world with his memories intact. Though he was surprised by the existence of magic and the strange species in that world, he quickly adapted to his condition as a newborn and took advantage of it. He acquired special magic and gained a massive amount of strength thanks to his rigorous discipline, in order to reach his goal: Resume his career as a teacher which he left halfway through in his previous life. This is the story of a man, who, based on the memories and the experiences of his previous life, became a teacher who travels through the world with his students.

This legend tells the tale of the protagonist, Kuroki summoned to protect the demon king from the hero.

Kuroki, unbeknownst to him, suddenly came to another world. He was summoned by the demon king, Modes. The territory which the demon king overrules was in the midst of being destroyed because of the hero. He had no choice but to bow before Kuroki.

“Please save us, Savior Dono!”

Kuroki nodded, deciding to save the demon king due to an unexpected turn of event.

(Immortal Cultivation+Meeting a Perfect Match+Shura Field+Invincible Flow+Upgrade Flow+Playing the Pig and Eating the Tiger)—What is cultivation? When I think back to the past, cultivation was, as a baby, kicking my legs over and over again—kicking on the bed, kicking on the ground, kicking every single day. Cultivation was, as I grew older, swinging my sword again and again—slashing here, cutting there. Cultivation became, as I matured, the repeated harmony of yin and yang—busy tonight, and busy again tomorrow night. … My name is Xu Ming. I have never cared about the stipends given to me by those sect masters, noble ladies, or princesses.I said to her, “Can you give me a room closer to yours? I need to report my work to you at any moment.” Even if I accomplish nothing every day, I just want to watch how she cultivates. I…

No family, no power, no inheritance — a thrice-lacking protagonist. Just how will the seed rise up from the mud?

- Description from Novelupdates