Toaru Majutsu no Index - Volume 13, Chapter 6: Streets Beaten by the Cold Rain. Battle_Preparation.

The mountain village boy accidentally got the fairy gourd, and he rose step by step to cultivate immortality and longevity.

Ripe elixir, mortal flow.

- Description from novelbuddy

Zhou Xu, an interstellar military counselor, was selected by the “will not die in peace” system, and as a result, even though he had a great fear of pain, began his journey as villain, forced to die a terrible death over and over again. Zhou Xu, after going through a hundred worlds and dying a hundred times, finally changed. He wanted to retaliate!

Leon Walker, the world-weary genius of modern world, is killed in a freak accident — the notorious Truck-Kun makes an appearance. Death is only the start. He finds himself reborn in a new world, Galvia, reincarnated into the strong but failing body of Duke Leon Moonwalker, one of the only three Grand Dukes of the Moonstone Kingdom, a minor but strategically powerful kingdom within Galvia. As Leon attempts to understand his new world, a strange voice resonates in his head. “Supreme Spouse System Activated.”.

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