MTL - Feng Jue of Ancient and Modern - Chapter 9 great nephew

In the current era, people are either normal or are with ‘special abilities’. Cases related to the latter are managed by the “National Anomaly Prevention and Control and Special Species Management Office”.
Qi Zheng Emperor Sheng Lingyuan was reawakened a thousand years later after he jumped off the fiery Red Lake. Xuan Ji, a newly transferred staff to the Reparations Division of the Deviant Control Office, finds himself having to clean up the mess of a revived tyrannical monarch. The two seemed to share something from the past, but unbeknownst to them, a big conspiracy is stirring right underneath their noses that could destroy the balance of the world.

- Description from Novelupdates

Meng Yu’s Golden Finger was called “Corruption Strengthens Tenfold, Becoming a Demon Strengthens Hundredfold,”both extremely fierce and convenient to use. “Those who consume Qi are divine and long-lived, constructing the foundation of ten-thousand generations, and on the day they achieve immortality (’Dan’), they break the self of yesteryears (’Infant’), transforming into heaven and earth, thus becoming immortals.” This is the eternally correct tale of the Qi Refinement Practitioner Meng Yu’s grand cultivation adventure.

Yuki, a high school junior, dies heroically saving a stranger and is reincarnated in a fantasy world, where he discovers an extraordinary and unrivaled power.

This is a story of a man named Kim Sae-Jin, who one day, inexplicably wakes up as a Monster. Follow him and his perilous journey to get his life back as a Human, all the while growing even more versatile as a Monster in the process. This should be… complicated, somewhat.