Rebirth in One Piece as Deputy Captain - Chapter 588

While the demons were rising, mankind was about to become extinct. Six temples rose and protected the last territory of mankind.

A young boy joins the temple as a knight to save his mother. During his journey full of miracles and tricks in the world where temples and demons are antagonistic to each other, will he be able to ascend to the strongest knight and inherit the sealed divine throne?

- Description from novelupdates

He was a Martial Arts prodigy, a master of using poisons!He descended the mountain to search for his junior sister but unexpectedly became the School Beauty's bodyguard… From then on, a demon-like student was born! He pursued the School Beauty, took down bullies, and was revered as the supreme ruler of the city! He cultivated Ancient Martial Arts, annihilated formidable enemies, and swept across all directions to ascend to the Peak.

Elijah is dying.Afflicted with terminal cancer, his days are limited. So, after deciding to spend his remaining time with family, he boards a plane and sets off for the Pacific Northwest to be with his sister. However, along the way, Earth is transformed by the touch of the World Tree, injecting it with magical Ethera and connecting it to a much larger universe via the System. More importantly for Elijah, though, it also causes his plane to crash, stranding him on a deserted island. But all is not lost, and through the System, he gains the Druid archetype that may be enough to allow him to survive a hostile environment filled with powerful monsters as well as more mundane threats like starvation and the elements. Meanwhile, his sister, Alyssa, must survive the collapse of civilization as she knows it while trying to keep her family safe. Follow them as they try to navigate an apocalyptic world full of game-like stats, classes, and threats aplenty.

Leon and his father, Artorias, are the last remaining scions of a once powerful and illustrious family of lightning mages. After barely surviving an attack that destroyed their home fifteen years ago, they now live in the dangerous Northern Vales, a desolate wilderness far away from civilization. But those who want them dead are strong, patient, and relentless, and it is only a matter of time before they find the two they seek...