Book Eater - Chapter 360: Seimei’s Legacy (1)

220 years ago, in the legendary Dragon Demon War, Hero Azell ended the war by killing the evil Dragon Demon race’s King Atein. He was able to save the population, but as a consequence, he was cursed. While he was dying, the high magician suggested a gamble that might save his life.

Three thousand years ago, Bai Qiuran, who was extremely talented, was accepted as a disciple by the founder of Qingming Sect, and the young Taoist began to cultivate the path of immortality.

A con-man and policewoman that are legends of their respective professions suddenly die, then are reincarnated and return to the Tang Dynasty of ancient China as little kids.

Accepting their second chance and forced restart, they choose to use the modern knowledge and specific talents that they have in their previous lives in order to live comfortably in this new life.

- Description from Novelupdates

This is an alternate history. Decades after the fall of the Eastern Jin Dynasty, the world is still in chaos, although there is hope for reunification.