Elf Tensei Kara no Cheat Kenkoku-ki - Chapter 13Book 3: -2: Where Did I Go Wrong?

The male lead is devoted, the female lead is passionate, childhood sweethearts, everyday family matters, the ordinary daily life of nurturing a wife.

Lynn embarks on a fantastical journey to a realm reminiscent of medieval Europe, yet infused with extraordinary magical abilities. Here, the very fabric of reality is governed by the enigmatic theory of magic, which harnesses and manipulates fundamental particles.

In this world, wizards, who are essentially scientific spell casters, find themselves locked in a relentless battle against the oppressive Church.

However, when Lynn arrives, a harbinger of an otherworldly Renaissance, the tides of change are set in motion, unveiling a thrilling tale of discovery, intrigue, and the struggle for enlightenment.

Intelligence without ambition is like a bird without wings.

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