How Harry Potter reacted when being punished in the Forbidden Forest?

For their punishment with Hagrid, Harry, Hermione, Neville, and Malfoy divide into groups and search the Forbidden Forest for unicorn that has been wounded. Harry and Malfoy come across the dead body of the unicorn just as a mysterious cloaked figure begins to drink its blood. Malfoy runs away in terror, but Harry remains immobile, incapacitated by the sudden pain that he feels in his scar.