6-sai no Kenja wa Hikage no Michi wo Ayumitai

"I defeated the demon king...?"
The strongest sage reincarnates as the six-year-old Eddie Radcliffe. Despite his inherited memories and magical prowess making him stronger than he ever was, he's determined to live a peaceful life. Contrary to his wishes, he's now tasked with saving the world from behind the scenes and is trusted as a ruler in the dark.

(Source: MangaUP!)