Looking for a manga

This was ons of my first ever mangas that i read. So my memory isnt that good about this manga.

I remember it is about a boy (black hair) and his mom said that he was a Demon baby or something at birth. I guess the dad said that he was a demon so the mom though the baby will also be a demon?. But one day the boy walked past or played near a pond and he fell in, and he transported to another world. And in this world he was a Demon king... or something. But he had someone, like a solder who kept him safe.. his name was something like David... but this 'David' tried to save the Demon boy and he lost his arm. And everyone thought 'David' died. But later on he came back alive and ofcourse, without his arm. There was also a blond boy that was friends with the 'Demon boy'. Thats all i can remember.
If anyone can help me find this manga I will appreciate it so much!