One Piece: Supreme Check-In System - Chapter 13: Check-In Amazon Lily

Warden wakes up in a wilderness with a hole in his mind, deprived of all his memories. Lost, he found within himself an aptitude for combat and magic.On happenstance, he rescues a pair of youths and an entire village, pushing him into a renowned academy of mages and warriors alike to become a teacher.Warden adventures across the divided land of Ezim, determined to unravel the mysteries of his past. However, the task becomes incredibly hazardous when his own path—the ruinous power of Void that runs through his veins—holds a connection to the Traitor Gods, who threaten to cast their shadow upon the world once more.

A teenager who unexpectedly gets transported to the modern world from ‘the world of the immortals’ . He finds himself in the body of an ordinary guy. He wants to go back to his world and for that, he will have to practice to get stronger and investigate the reason why and how he crossed through to this world.

The aliens came with hostility.
Facing the invasion of aliens, human beings are weak like ants.
So this is the end, the real end.
But there is resistance when there is an invasion.
For some, this is not the end, it is war.
So this is war.
Real war!

- Description from Qidian

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.