Honkai Impact 3rd : This is My Revenge - Chapter 48 - 047 : Pardofelis

The Jiuzhou Continent contains an era of chaos and fiendish demons, haunted by mysterious beings and fraught with feuding martial families, demonic sacred grounds, and endless struggles between various powers. Conflict is a constant throughout the world.Within such darkness, Gu Chen opened his eyes to this reality.At the same time, he discovered that as long as he slew these fiendish demons and ghosts, his cultivation strength could continually elevate.Killing a Panic Level Demon yields a decade's worth of cultivation.Slaying a Ghost Level Fiendish Ghost brings thirty years of cultivation....Eventually, one day, staring at the endless swarm of fiendish demons and world-renowned grandmasters in his path, Gu Chen chuckled lightly, “Are you able to withstand this fist of mine, charged with 1600 years of cultivation?”

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

A new world, filled with mysteries and deadly dangers. New faces, hiding their true intentions. Horizons beyond his wildest dreams. The world of Ice and Fire has opened its gates to him—there is no turning back.Born into a family of ordinary knights, he has no powerful benefactors, no secret knowledge of what lies ahead, and only a vague understanding of this world’s ways. But one thing is certain—he will never stop moving forward. Intrigue, betrayal, enemies—mere obstacles on a path he will carve with his own hands. His life will become a legend, but what kind of legend? That is for him to decide.The first step has been taken. Welcome to a journey that will shape the fate of Planetos.

[The Demon King is Dead], a story in which the Demon King got immediately killed in the Prologue. I became the ‘Demon Prince’ of said novel.

…Help me.