Elf Tensei Kara no Cheat Kenkoku-ki - Chapter 14Book 3: -2: Kuu’s Brilliance

As a wandering soul trapped in endless cycles of reincarnations, Claus must decide what to do in his current life.

When Li Xianyu was 20 years old, he received his father’s inheritance, an 18-year-old great-grandmother who was incredibly beautiful.

Li Xianyu: “Great-Grandma, why are you so young?”

Great-Grandma: “That’s because I died when I was 18 years old.”

- Description from Novelupdates

In a world dotted with World Entrances to a demon world, demons have invaded humans for centuries. Humanity has united and one of the most ancient sects in the world, the Archean Mountain Sect, has set up an entire education system in the form of Dao Academies and defenses at the World Entrances.