"'Potter, you cannot win against me!' she cried. He could hear her moving to the right, trying to get a clear shot of him. He backed around the statue away from her, crouching behind the centaur's legs, his head level with the house-elf's. 'I was and am the Dark Lord's most loyal servant, I learned the Dark Arts from him, and I know spells of such power that you, pathetic little boy, can never hope to compete'--- "
---Harry Potter and the Order of the Pheonix, by J.K. Rowling, ch. 36, p. 811