Harry Uses Sectumsempra on Draco
Pages 522-523
During the heated duel, Harry instinctively uses the Sectumsempra spell on Draco Malfoy. The spell causes deep, bloody gashes to appear on Malfoy's face and chest, leaving him severely injured. Horrified at the consequences of using a spell he didn't fully understand, Harry tries to stop the bleeding but is unsuccessful.
