Mirror Sword and Shadow Prince

When the heir to the empire comes to Mino, the lives of young Oguna and Toko change forever. Oguna is drafted to become a shadow prince, a double trained to take the place of the hunted royal. But soon Oguna is given the Mirror Sword, and his power to wield it threatens the entire nation. Only Toko can stop him, but to do so she needs to gather four magatama, beads with magical powers that can be strung together to form the Misumaru of Death. Toko’s journey is one of both adventure and self-discovery, and also brings her face to face with the tragic truth behind Oguna’s transformation.

Source: Haikasoru