Yasashii Ryuu no Koroshikata

Two opposing worlds exist—yang, home to humans, and yin, home to mythical beasts. Long ago, warring clans established a lasting peace by separating into these realms. A mythical beast king, in love with a human queen, gave her his sword and promised to protect her and her descendants in times of need. As a third war looms, a swordsman with the blood of the former queen meets the distant ruler of the mythical beasts, setting events in motion.