Disclaimer; Not a single thing is mine!
Sparta, a legendary demon knight, is a mystery that has plagued humans and wizards alike, First he fell for a human woman, and he betrayed his fellow legions to protect the human realm from there constant attacks. He loved his wife for centauries, and had a set of identical twin boys, named Dante and Vergil. But there was another, one that was a full demon, from before Sparta had come to the human plain.
But there was a problem with his first son, he had been born with a soul, and as the demon law stated, he was ritually sacrificed, yet Sparta managed to safe the soul, and he placed it into a vessel in the human realm, this vessel was James Potter.
As he only had a soul of a demon, James never felt the pull of his demonic blood, so he looked, lived and loved like a human, and his son Harry was born latter with the love of his life, Lily.
Yet there was a problem, as magic is unstable, the magic from both parents activated the blood in Harry, making him slowly become a demon, but this process should have taken around 50 years, as it was a complete accident that it had occurred and no one in either realm knew about it or its process.
But one single individual cursed Harry with an accelerated rate of blood consumption, and killed both his parents and his human side in one fateful evening, Voldermort. The killing cursed that everyone excepted as doing no harm was in fact both a key to Harry's demonic body, as well as the trigger to all his normal life to come.
The curse killed of his human part, but the magic that was flowing through him kept Harry alive, long enough for the demon blood to resurrect the dead cell in it's image.
As of yet, Harry hadn't felt or come into contact with anything of the demon world, but as he neared his 16th birthday, his day of majority, harrys life suddenly took a shift, taking him away from the school he loves most, and the family he hates.