The Fourth Primogenitor-That's the world's strongest vampire that should only exist in legends. Accompanied by twelve Kenjuu and spreading calamity, this phantom vampire appeared in Japan. For the observation and obliteration of this Fourth Primogenitor, the government and Lion King decided to dispatch an attack mage known as a Sword Shaman. However, for some reason, the one chosen for the observation was a apprentice Sword Shaman girl, Himegari Yukina. Wielding the strongest anti-primogenitor spirit spear, Yukina arrived at Demon District, Itogami City. What is the true identity of the Fourth Primogenitor, Akatsuki Kojou, she encountered over there!?