Demon Playground

(Think of it like devilman, but I'm not try to base it off it as you might see a few references from the anime in here.This will have blood,guts,and sexual content so read please with this in mind)
Long time ago demons use to rule the world as they absorb,and kill each other to get stronger. But one day humans show up. An they started to change as they got smarter and made stories after the demons. But most demons went into hiding as they blended in with humans as they grow overtime. But...

Then their is demons that pick a kid, when they are born, and gives them their powers. But they don't gain their powers intel they are 15. They get stronger as their demon allows them to have more of their power or absorb another demon.

There is a school for these kids chosen by a demon. That the demons hidden in the government send these kids to to test they abilities. An at this school their is no rules as it is survival of the fittest.