Book 6: Harry Potter and the Half Blood Prince

Rufus Scrimgeour replaces Cornelius Fudge as Minister for Magic because of Fudge's previous inactions against Lord Voldemort Voldemort and his allies begin to act openly, causing widespread chaos, paranoia, and depression in Britain. Severus Snape makes an "Unbreakable Vow" to Draco Malfoy's mother that he will aid Draco in his first mission as a Death Eater: to kill Professor Dumbledore.

Security measures have been increased at Hogwarts. Snape is given his coveted Defence Against the Dark Arts post while Horace Slughorn returns from retirement to take his Potions post. Slughorn lends the unprepared Harry an old textbook marked as the Half-Blood Prince's. Its handwritten notes help Harry to outdo anyone, even Hermione, in Potions.

With Slughorn's reluctant help, Dumbledore shows Harry details of Voldemort's past. While two of Voldemort's Horcruxes have already been destroyed (Tom Riddle's diary by Harry and Salazar Slytherin's ring by Dumbledore), Dumbledore believes that Voldemort has four more that must be destroyed before Voldemort can die. Dumbledore and Harry set off to retrieve one (Salazar Slytherin's locket), but Dumbledore is heavily weakened by the potion guarding it.

They return to find the Dark Mark over Hogwarts and Death Eaters attacking students and teachers. As they investigate, they are surprised by Draco. Dumbledore paralyses Harry, who is invisibly cloaked. Draco reveals how he let the Death Eaters in. Other Death Eaters soon arrive and urge Draco to fulfill his mission, but he hesitates. Snape arrives and kills Dumbledore with Avada Kedavra. Harry pursues Snape, who identifies himself as the Half-Blood Prince before escaping.

Professor McGonagall succeeds Dumbledore as head teacher, but she and the other teachers fear that Hogwarts may have to be closed because of the Death Eaters' attack. After Dumbledore's funeral, Harry decides not to return to school so that he can devote his time to destroy the remaining Horcruxes and to kill Voldemort.