The Blackflame Knight DxD

In a world of the supernatural, where humanity is naive to its existence, Issei Hyoudou would have a bad day of the fact that the first girl that he had ever loved, suddenly killed him through a stab of the spear made of pure light. Except that it didn't and he was saved by this mysterious black knight that came out of nowhere. The question is: Who is he? And why is he terrifying?