Tangled in Moonlight: Unshifted - Chapter 343: Ava: Prototype Progress

I have experienced all the eight sufferings of this world.

I do not ask for nirvana, but for you.

Wild and hard-to-tame demon (gong) x cold, ascetic and childish immortal (shou)

- Description from Novelupdates

Ronan was just another overworked office drone until a sudden death transported him to a vast, unfamiliar fantasy world—alone, invincible, and armed with absurdly powerful abilities from a game he once played. But reality isn't as simple as a game, and after days of wandering the unending Nim Forest without finding civilization, his optimism begins to fade.Just as isolation threatens to wear him down, he encounters Frieren, a quiet elf girl hiding in the bushes. Their meeting marks the first step on a path neither of them fully understands, in a world that seems empty, yet full of mystery.

One day, when a novelist woke up from a restless sleep,he realized that he had transformed into his 12-year-old self.

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