The Tyrant's Pet - Chapter 780 Are You Done?

Li Hua's reign as the twenty-first century's deadliest assassin ended in fire and betrayal. Her own sister, Li Min, turned their elite force against her, leaving nothing but ashes where a legend once stood.In the moment of her death, Li Hua's soul tore free from its earthly bonds, hurtling across the boundaries between worlds until it anchored itself in a new vessel in a realm of ancient martial arts and mystical cultivation. Through this impossible transition, her inner space remained intact—and with it, Little Firefly, her spirit companion.Here, where qi flows like rivers and mountains bow to those who master their cultivation, Li Hua will forge herself anew. No longer content to be merely an assassin of mortal flesh, she sets her sights on ascending to immortality itself—a journey that will shake the very foundations of heaven.

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

Suddenly a high school student Subaru Natsuki has been summoned to another world on the way back from the convenience store. With the biggest crisis of his life being summoned to another world and no sign of the one who summoned him things become worse when he is attacked.