Demons were ignored and shunned by humans for millions of years. Having enough of it, they went up and wiped out almost all of the human population. As a sign of peace, to satisfy the demons and keep humans from dying out, each year, only a thousand women would give birth to twins, one human and one demon. After centuries, there have been no huge problems between the races. Unless you count the Livah family's disaster, of course.
I was given permission by Fandom-Lover18 to adapt their story. I just changed the names and will change a few other things. We came up with the same idea, they just wanted to write a fanfiction off of it. They write the main body of the story, and I just adapt it to fit what I write.