My Descendant Begged Me To Help Him Just After I Became A God - Chapter 46 - : Certain Death, Divine Weapon Descends from the Sky

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

He became a teacher at the best magic academy in the empire.
….For being mistaken for someone else.

The story is about a man who’s opposed to the now-common practice of implanting false memories, but is one day given memories of a fictional childhood friend. This fabricated happiness torments him, but then, to his surprise, he meets her – a girl who shouldn’t exist.

In childhood, they were neighbors. Xie Zhen always bullied Li Yu.

Who told him to be as beautiful as a girl?

After parting for many years, they met again in the capital. With a tall stature and heroic bearing, he seemed like a completely different person.

Not only that, but his identity had also risen to that of the sixth prince.

Hearing he wanted to marry her, Xie Zhen was almost scared silly:

Σ(っ °Д °;)っ What’s his intention? Taking revenge on her?

- Description from Novelupdates