Think Bleach, but without the shounen and powerlevels (themes).
Think romance, but without drawn out staring and not AS much tsundere (more socially retarded) as the usual.
Think "crimson gothic" because that's the "aesthetic" feel to the style/world/etc..
The main antagonists are beings that can't live without "power of existence" and don't produce their own, humans produce it naturally, thus the dilemma of the world's plot backing.
The rest is made up as it goes along, really, there is no clear knowledge or depiction of what the antagonists are doing, it's mainly a story about the characters.