Rebirth Begins with Rejecting my Childhood Friend - Chapter 129 - Good Friends Relationship.

Before Li Daniu had time to be excited about being a prince who was about to ascend the throne, he learned that his country was about to be submerged by sea water.

“Can I wear it back?”

- Description from novelbuddy

A story of Gin, a young man who lost his memories before the age of 17 and became a magician. Living a quiet life running a cafe while researching magic, he is suddenly thrust into an unknown world. Moving between Earth and a realm filled with danger and secrets, Gin must rely on his knowledge, skills, and magical prowess to navigate the perils that arise. Ruthless and determined, he will stop at nothing to uncover the truth behind his lost memories and the mysteries of the world, using every tool at his disposal to rise above all

Kai of the Sorcerer’s Tower is one of the last mages left, barely surviving in a world ruled by creatures of the dark — where the era of mages ended, and most of the lands got contaminated with dead mana. To save his world and fulfil his master’s last wish, he put himself through a forbidden ritual, risking his soul being ripped into pieces to make his way into the past. In an era with abundant mana — an era yet to enter the Golden Era of Magic. Things go wrong, of course, as they always do, and he finds himself thousands of years before the world fell to mana contamination — in the body of a young lord, Arzan. All is not well, though, as the young lord was murdered by someone right at the cusp of a beast wave threatening the territory, while larger threats loom on the horizon. There was a lot to do for Kai to regain his former strength in this time of primitive technology, but then again, he might just be able to change that.