Okay, my two cents.
An ancient feud between humans and demons has been waging in our world without the majority of humans knowing this. A human girl is thrown in the middle of this conflict and through her, the readers experience this chapter of human history, but in reality it's her journey to adulthood, finding and accepting herself and doing the thing she would have never done in the beginning.
It's a story about how our actions can shape the world, and the world shapes us at the same time. A story about loss, acceptance, discovery, tragedy which is all-about-you and a-lot-bigger-than-you at the same time.