Oukoku no Kagi

After the king and his eldest son die, the kingdom of Landor is left without a ruler. Asta, the last surviving member of the royal family, stands in line for the throne.

However, 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?