With the arrival of the Primogenitors, the Electoral War for Itogami Island plunges further into chaos. Shio and Yuiri head to the island intending to use Twelth Primogenitor, Avrora, to put an end to the conflict, only to come face to face with Second Primogenitor, Fallgazer. While Yukina rushes to the island to save the three, elsewhere Kojou is suddenly overcome by inexplicable hunger and thirst, which is apparently tied to the Beast Vassals that resonate with the Primogenitors. Just what will Kojou have to do to regain his strength and stop the Beast Vassals' rampage?