Demon-Hunting Genius Mage - Chapter 91

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

Julian Sterling was the young master of the Sterling family, trained since childhood to become the most perfect heir to the Sterling Group. With his dashing appearance and a height of over 1.8 meters, Julian Sterling was almost regarded as the image of an ideal husband: wealthy, handsome, and talented. But who could have known that he was a person harboring a secret wish to become one of those small, cute boys who get carried and pampered like the bottoms in Boys' Love novels.That is right, it was Boys' Love - because our President Sterling only loved men. Perhaps it was because life in the Sterling family was too exhausting, with too much pressure, that he always yearned for someone to come and take him away.Maybe it was because the heavens had eyes, or his deceased mother pitied him too much, but finally, during a business trip, Julian Sterling lost his life in a car accident at the age of twenty-seven. Fortunately, his soul transmigrated into a Boys' Love novel - becoming the real young master with the same name and surname who had just been found and brought back to the Sterling family - a person about to be married off to the head of another tycoon family.Julian Sterling could not help but lament that everything was truly miserable, but luckily... he had turned into a small and cute boy now!

“Are you my knight?” The silver-haired youth looked at the blonde girl before him and asked, his cheeks flushed red. This was their first encounter when he was only ten years of age.

“Are you my bride?” He held her in his arms, embraced the girl who had accompanied him for ten years straight. He was sixteen years of age.

“Are you my king?” He knelt before the throne, smiling foolishly as he looked at the girl holding the sword victoriously. At this moment, he looked like the boy he was when they first met.

“I’m fine without having eyes for you. The entirety of Camelot is for your taking.” Smiling soft and gently, he gave his eyes to the girl who lost her eyes due to the curse of the red dragon. He doesn’t regret it, he thinks. He was twenty years of age.

“So… Arthur… Artoria Pendragon, are you willing… are you willing to spend the rest of your life with me?” His voice was soft and clear as he reached out to touch her hand.

This was their legend.

- Description from Novelupdates

I became the childhood friend of the protagonist in a novel. The childhood friend who died from taking drugs.