(This description was taken from my deviantart post about this story from years ago)

Basically the story was about two worlds that were connected by an interface, and there were “angels” on one side and “demons” on the other, and they were constantly waging war, trying to get control of both sides. Only people who were born of both bloods could open the gate between them, and they are considered to be outcasts. However, this little boy was the child of the king of the demon world and a minor royal woman of the angellic world, and, being the only son of the king, his true identity was hidden for years until he was captured as a prisoner of war on the angellic side, and they figured out what he was.

He spends a lot of time on the angellic side, seeing their point of view, and decides to switch sides (though this was a decision he had thought of throughout his life). This causes major catastrophe to happen. SO much catastrophe that the war opens a dimensional rift between the two, and leaks into our reality. It turns out that all of these angels and demons are the souls of people who died in our universe, and the people who are both are actually souls of people who are in comas. He wakes up in his true form on earth, and the culmination of the both sides of dark and light grants him massive powers in our world, threatening to destroy it. It turns out that the alternate universe is made of water, and the light and dark sides are simply how light is reflected in the liquid. He creates massive vortexes using water, marking the apocalypse on earth (a great flood), and the “angels” and “demons” come through the other side to try and stop him together, marking the beginning of the great war that was predicted in the bible.