"My theory is that the ward your mother put on, which is that Futhark rune on your forehead, wasn’t fully finished. It was enough to protect you, but the rebound from his ritual must have caused his soul fragment to travel to the rune, creating a scar." There were several other possibilities, but I was not privy to the actual ritual for creating a Horcrux. I could never touch that foul book to read it. But the concept of having to murder an innocent in cold blood, would sever the soul which could be used to make a Horcrux. "It's safe for the time being, but I want to get it out before anyone tries killing Voldemort." 