There is a legend that tells of how the world was almost destroyed thousands of years ago by the vampire queen after she unleashed her latent powers. Despite their efforts, the humans were unable to defeat her, and thus resorted to sealing her away until the time came that they would be powerful enough to destroy the seal and kill her. However, the humans are not the only ones after the seal. Having cast aside his kingdom and betraying his own race and the dhampirs (half-vampires), the vampire king is persistently searching, even to this day, for where his queen has been sealed away so he may break the seal and free her before the humans do. Fearing that once the king and queen are reunited, they shall continue to destroy the world together, the humans and dhampirs hunt the king, using any method possible.

(Source: Anime Source)