18+ Minerva no Kenshi

The Kingdom of Doria enjoyed an era of peace, but that changed when King Baasen was overthrown and murdered by his closest confidant, Randis. To secure his own legacy, Randis ordered the execution of the late king's only son, Prince Sho. When the guards sent to kill Sho found him playing with Randis' daughter Diana in the castle garden, he attempted to flee, but was pushed off a waterfall in the ensuing struggle and pronounced dead.

Twelve years later, Diana—now a princess—is set for a political marriage with Prince Dunan of Guptia. Unable to accept this responsibility, she disguises herself as a man and flees the kingdom. By a twist of fate, Diana is soon captured by a band of nomads, who plan to make her their slave. However, before she submits, a grown-up Sho happens upon the nomads' camp. Sho negotiates with them for her ownership and their assistance in reclaiming Doria. Succeeding, the two childhood friends reunite and begin the journey back to their homeland, all while tackling personal and political issues along the way.