The Key to the Kingdom

A new series debuts from renowned fantasy manga artist Kyoko Shitou! After a king and his eldest son die, the kingdom of Laudere is left without a ruler. Asta, the last surviving member of the royal family, stands in line for the throne. But before he can claim it, he must fulfill a quest: find the legendary "Key to the Kingdom." Both ancient beasts and a variety of contenders — including a princess, a dimwitted military captain, and a villainous aristocrat — imperil his journey. Someone will find the Key, but will it be Asta?

source: https://www.mangaupdates.com/series/q9kawc2/oukoku-no-kagi