Angels, Demons, and Humans.

Put in charge of the affairs of the Heavens were Seven angels known to man as the Archangels, Originally seven in number, an eighth archangel was added to the mix.

Love is a concept not foreign to the heavenly beings but was forbidden between them and other creatures.

But that Law was defied. 

The eighth Archangel, fell in love with a human. He forfeited his role, power and title as an Archangel to become flesh- a man. But some of the heavenly hosts were unhappy and secretly plotted to end his life. Then, he had a son.

And that was when the hosts unleashed war on mankind.
The war was brutal. Lives and lands were lost. Families, friends and loved ones were separated from one another. 
Eventually, the eighth was found out. Or rather, he came out. From the compassion of his heart he sacrificed his life in order to save humanity- including his own son. 

The mother of the child fled. To preserve the life of herself and the boy, she escaped to another realm, the other side of humanity, away from mythical beings and heavenly bodies, a place called Earth.

There, she and the boy were a safe. And they could live a normal life.
Fifteen years later, the boy came to know the truth behind his father's death. 
And he swore, to wage war on the heavens and to avenge his father.

Chapter 149 - Chapter Extra III