<P> A new character appears when former Hogwarts Potions master Horace Slughorn replaces Snape, who assumes the Defence Against the Dark Arts post . Harry suddenly excels in Potions, using an old textbook once belonging to a talented student known only as "The Half - Blood Prince ." The book contains many handwritten notes, revisions, and new spells; Hermione, however, believes Harry's use of it is cheating . Through private meetings with Dumbledore, Harry learns about Voldemort's orphaned youth, his rise to power, and how he splintered his soul into Horcruxes to achieve immortality . Two Horcruxes have been destroyed--the diary and a ring; and Harry and Dumbledore locate another, although it is a fake . When Death Eaters invade Hogwarts, Snape kills Dumbledore . As Snape escapes, he proclaims that he is the Half - Blood Prince (being the son of a muggle father and the pure - blood Eileen Prince). It now falls upon Harry to find and destroy Voldemort's remaining Horcruxes and to avenge Dumbledore's death . In a 2005 interview, Rowling stated that (after the events in the sixth book) Harry has, "taken the view that they are now at war . He does become more battle - hardened . He's now ready to go out fighting . And he's after revenge (against Voldemort and Snape)." </P> <P> This book also focuses on the mysterious activities of Harry's rival Draco Malfoy . Voldemort has coerced a frightened Malfoy into attempting to kill Dumbledore . During a duel in Moaning Myrtle's bathroom, Harry uses the Half - Blood Prince's spell, Sectumsempra, on Malfoy, who suffers near - fatal injuries as a result . Harry is horrified by what he has done and also comes to feel sympathy for Draco, after learning he was forced to do Voldemort's bidding under the threat of his and his parents' deaths . </P> <P> In Harry Potter and the Deathly Hallows, Harry, Ron, and Hermione leave Hogwarts to complete Dumbledore's task: to search for and destroy Voldemort's remaining four Horcruxes, then find and kill the Dark Lord . The three pit themselves against Voldemort's newly formed totalitarian police state, an action that tests Harry's courage and moral character . Voldemort's seizure of the Ministry of Magic leads to discriminatory and genocidal policies against Muggle - borns, fuelled by propaganda and fear . According to J.K. Rowling, telling scenes are when Harry uses Cruciatus Curse and Imperius Curse, unforgivable curses for torture and mind - control, on Voldemort's servants, and also when he casts Sectumsempra on Draco Malfoy during the bathroom fight in the sixth book . Each time shows a "flawed and mortal" side to Harry . However, she explains, "He is also in an extreme situation and attempting to defend somebody very good against a violent and murderous opponent ." </P> <P> Harry experiences occasional disturbing visions of Draco being forced to perform the Death Eaters' bidding and feels "...sickened...by the use to which Draco was now being put by Voldemort," again showing his compassion for an enemy . </P>

How old was harry potter when he left hogwarts