Mimizuku to Yoru no Ou - Chapter 4 - Deliverance

Lee Se-Hoon, the blacksmith of humanity’s last desperate united force, fought against the demons until the very end. However, the result was mutual destruction, with the world coming to an end. The moment he took his own life over that futile conclusion—

“…What is this?”

He had regressed to his days as a candidate at the hero training institution, Babel Academy. Having obtained a second chance, he swore to use anything as material to gain victory this time. Anything at all!

An upsurge of emotion, infinite fantasy, the wind waving Melaleuca, undefeated young blood! After the rebirth of Nie Yun lucky to have the ability to change the number of dantian law formula, others with a dantian practice, you use ten, one hundred!

- Description from Uukanshu

Fate had never been kind to Ji Ning. Wracked by illnesses and infirm his entire life on Earth, Ji Ning knew early on that he would die as a teenager. What he didn’t know was that there really was such a thing as life after death and that the multiverse was a far larger place than he thought.

He became a teacher at the best magic academy in the empire.
….For being mistaken for someone else.