S1·E1 Son of Destiny

It's the 21st century and Japan is ruled by a tyrant right after a massive holocaust. A wise seer named Nagi predict that a savior is born within a small, remote village called Byakko Village. Fast-forwarded it to a few years and Tatara, also known as the Child of Destiny (the said savior) has already grown up to be a teenager, along with his twin sister named Sarasa. Their peaceful lives were shattered when the ruthless Red King's army marched into the village and killed Tatara. Determined to help her village people and avenge her family's death, Sarasa took on Tatara's identity before leading her villagers to safety.