Storm

Heaven and Hell were once enemies since Satan had been the one to betray God and create his own world where demons were made; but once God made a little mistake and created monsters that if not fought then they could easily destroy the world, he begs Satan to help him out.

The Devil and God then think and think and think before they realize that if you were to morph a demon and a angel you would create a god who has the powers of both of theirs, so every 1,000 years there is a ceremony that chooses the next demon and angel to go down to earth and protect it from the monsters that are created every single day.