Monsters, or otherwise known as 'impurities' are creatures created by darkest desires of a person. When a
sudden amount of monsters started to consume Granvania, Itsuki was called upon to battle against them, with the help of star children. All monsters take alternate appearances, some looking similar and some very different. The boss of a labyrinth usually maintains an individual appearance compared to the other monsters and are naturally the strongest monsters of the labyrinths. Each boss rules a labyrinth and it's within Itsuki's power to defeat them to restore peace and harmony.
Is the Japanese title for the monsters (impurities) which are otherwise known as 'demonic creatures'.