Its the modern day but unlike reality, their is a thing called Mana that exists, it permeates everything in various levels in three forms, Positive, Negative and Neutral. Monsters are creatures born from mana clumping together, the more mana and the more powerful it is the higher its rank,

some people are born with the ability to manipulate their own mana, they have innate abilities called a technique which is the expression of their mana. such people are called Mages and have been battling monsters for as long as anyone can recall, some clans exist which are families of mages as the ability to manipulate mana while it can occur in random people is mostly genetic.

Mages, monsters and anything to do with mana is graded from F- to A+ rank, with anything above A+ being called a Sage rank which simply means its such an overwhelmingly special and imposing force that its nearly impossible for anyone belo A rank to deal with. Monsters above a B- rank tend to be intelligent and self aware.

the current situation is that you are a student in Kyoto, Kaisaki High school, a senior in fact and you have been asked to stay after school to clean some things up, you are not a mage but neither of your parents are so you kept it to yourself to avoid issues.

However...while finishing your cleaning of Room 203 you are about to leave when you suddenly hear some noises down the hall and when you look down to see if it was someone else you gaze upon the large car sized monster, its dark purple skin, multiple limbs and mouths eagerly look for humans to consume....

But as your contemplating on if you should intervene you are interrupted by a black haired guy breaking through a window into the school who is riding a large bird like monster, his name is Damian Unaki, he is a C+ Rank Mage and his technique is called Subjugation which allows him to absorb any monster he beats to use as a summon for his own purposes however the limit to how many he has is currently 6. he engages in battle with the monster in the hallway.