The Beginning After the End

Saikyou no Ousama, Nidome no Jinsei wa Nani wo Suru?

After a mysterious death, King Grey is reborn as Arthur Leywin on the magical continent of Dicathen. Although he enters his second life as a baby, his previous wisdom remains. He begins to master magic and forge his own path as the years go by, seeking to correct the mistakes of his past life.

(Source: Crunchyroll)