MTL - Start with Batman - Chapter 133 shapeshifter

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

After the ‘100-Year War’ between the Necromancers and Priests, an all-time genius who’ll shake the power structure was born. “Am I a rare case or something? Do I have talent?” A potential beyond all imagination. He obtained his father’s undead army and entered the great Necromancer school of Kizen that divided the continent. Geniuses are geniuses, even when gathered among the elites. The research community was flipped on its head by the appearance of a new case. Professors wouldn’t leave him be alone, wanting to make him study directly under them. Officials from all over the kingdom and heads of organizations fidgeted over whether to scout him. “Professor! When can I make a Lich?” “Gimme a break. How talented are you? You’re crossing the line, honestly.” A genius among geniuses had appeared.

The Earth mutated and an entire city disappeared from its original world! Su Yi arrived in the Different World strangely with the whole city, where demons ran amok, dynasty hegemony. There was no peace of modern civilization, only the law of jungle! Fortunately, his phone also changed... Gods, demons, monsters, Buddhas, fairies, etc., were all pulled into his mobile phone for his use! I am the king of demons, and I am the Lord of gods!

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