“I knew too where Voldemort was weak. And so I made my decision. You would be protected by an ancient magic of which he knows, which he despises, and which he has always, therefore, underestimated — to his cost. I am speaking, of course, of the fact that your mother died to save you. She gave you a lingering protection he never expected, a protection that flows in your veins to this day. I put my trust, therefore, in your mother's blood.”
―
J.K. Rowling,
Harry Potter and the Order of the Phoenix