Chongfei Manual - Chapter 110

Two heroes of the game Legend of Heroes 2 who were rotten waters*.

He is a young man without a spirit root. It is believed that this denies him the chance at cultivation. However, a mysterious lady imparts a Supreme Demon Classic to him and from then on, he starts his path of cultivation.
He undergoes a total transformation to emerge as the most fearsome and powerful fiendish demon in his era, that even immortals and fiends are fearful of him, and saints are at his beck and call.
Mastering the Supreme Demon Classic is the turning point in his life. He is able to unleash his immense and divine power from within, illuminating the entire universe!

- Description from Novelupdates

Shen Yuan, a normal human who was living his life peacefully on earth died mysteriously, only to get reincarnated into the world of cultivation with an unknown bloodline so powerful that it even surpassed his chest system....“Hmm, looks like becoming an alchemist might be good. System, purchase the Alchemy knowledge.”[Ding!][Congratulations Host, for acquiring Heaven Refining Alchemy Sutra.]...Follow to see the journey of how Shen Yuan creates the strongest harem since the Primordial times to journey the three thousand world.

This story is centered around the man named Yi Ji-Hyuk, who has spent 1000 years in a Medieval Western fantasy-like world before returning back to the modern-day Earth. When he returns, he finds that his home is facing monster threats, and that humans with superpowers have appeared to protect the innocent people of Earth.