Oukoku no Kagi

With the king and his eldest son deceased, the kingdom of Landor lacks a ruler. Asta, the sole remaining royal family member, is next in line for the throne. Before ascending, he must complete a task: locating the fabled "Key to the Kingdom." His journey faces threats from ancient creatures and rival seekers, such as a princess, a bumbling military captain, and an evil noble. The Key will be discovered, but whether it is Asta who finds it remains uncertain.