Sectumsempra

Harry, still unable to enter the Room of Requirement, stumbles upon Draco Malfoy crying in a bathroom. Malfoy reveals he is terrified of his mission and fears Voldemort will kill him if he fails. A duel erupts, and Harry, in a panic, uses an unknown spell from the Half-Blood Prince's book: Sectumsempra. The spell slashes Malfoy's face and chest, causing severe bleeding. Snape arrives and heals Malfoy, then demands Harry's schoolbooks. Harry swaps his copy of Advanced Potion-Making with Ron's, but Snape finds the name 'Roonil Wazlib' inside and knows it is not Harry's. Snape gives Harry detention every Saturday for the rest of the term, preventing him from playing in the Quidditch final.