Harry Potter and the Half-Blood Prince

In the sixth book, Harry Potter and the Half-Blood Prince Harry enters a tumultuous puberty that, Rowling says, is based on her and her younger sister's own difficult teenage years. Rowling also made an intimate statement about Harry's personal life: "Because of the demands of the adventure that Harry is following, he has had less sexual experience than boys of his age might have had". This inexperience with romance was a factor in Harry's failed relationship with Cho. Now his thoughts concern Ginny, and a vital plot point in the last chapter is when Harry ends their budding romance to protect her from Voldemort.

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 using 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 the 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 - Harry's admired mentor is actually his hated enemy. It now falls upon Harry to find and destroy Voldemort's remaining Horcruxes and to avenge Dumbledore's death. In a 2005 interview with NBC anchorwoman Katie Couric, 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]."

Much of this book focusses on the mysterious activities of Harry's long-time rival Draco Malfoy who, it turns out, was coerced by Voldemort to kill Dumbledore, much against his will. At one stage he and Harry duel in Moaning Myrtle's bathroom and Harry uses the obscure Sectumsempra spell that he learned from the mysterious Half-Blood Prince's book and as a result Malfoy suffers near-fatal injuries. Harry is horrified by what he has done to Draco in the bathroom incident and even feels a twinge of compassion for his enemy when he realises that he was forced to do Voldemort's bidding under the threat of his and his parents' deaths. This shows a compassionate and forgiving side to Harry, even towards his enemies.

In Harry Potter and the Half-Blood Prince, Ron receives fair grades on his O.W.L. examinations. He passes seven subjects and only fails History of Magic and Divination, just as Harry does, but he receives no top scores of "Outstanding" like Harry (who received one) or Hermione (who received nine and one Exceeds Expectations). Ron, who has grown taller over the summer, attracts the attention of Lavender Brown. Harry, the new Quidditch Captain, picks Ron to continue as Keeper for the Gryffindor Quidditch team, over competing candidate Cormac McLaggen who is equally-skilled but has difficulty with teamwork and following orders. Upon learning Hermione most likely had kissed Viktor Krum, Ron performs increasingly badly at Quidditch, and thrown off by jealousy of his former idol, becomes unkind to Hermione. His low self-esteem is not helped much by his younger sister, Ginny Weasley, who after Ron reacts badly to finding her kissing her boyfriend, throws in the fact that of those in the group, Ron is the only one who has never had his first kiss. To bolster Ron's confidence, Harry pretends to give him Felix Felicis, a potion which makes the drinker lucky; believing he has actually taken it, Ron performs admirably and Gryffindor wins the match. However, this leads to a major row between Ron and Hermione: Hermione accuses Harry of helping Ron cheat, while Ron berates Hermione for having no faith in his abilities. At a post-game celebration, Ron kisses Lavender (though Ginny describes it as "eating her face"). Hermione, jealous and seeking retaliation, takes Cormac McLaggen as her date to new Potions professor Horace Slughorn's Christmas party, but he proves to be an egomaniac. After Christmas, Hermione continues to ignore Ron, stopping only to give him disdainful looks and occasional snide remarks. By now, Ron is visibly discontent with his relationship with Lavender.

On his birthday in March, Ron accidentally eats love-potion-infused Chocolate Cauldrons (actually meant for Harry). After being cured by Slughorn, he then consumes poisoned mead (which Draco Malfoy actually intended for Dumbledore). Harry saves his life by forcing a bezoar, a poison antidote, into his mouth, and Ron is transferred to the hospital wing. A panic-stricken Hermione arrives, forgetting her past anger. While sitting by his bed, Hermione, Harry, Ginny and the twins hear Ron mutter Hermione's name in his delirium. Conversely, Ron feigns sleep when Lavender visits him. Upon recovering, Ron and Hermione reconcile. A little while later, Ron and Lavender break up, much to Hermione's joy.

Initially, Ron does not support Harry's belief that Draco Malfoy is a Death Eater, a follower of Voldemort, but later is convinced. Before leaving Hogwarts with Dumbledore to recover a Horcrux, a piece of Voldemort's soul stored in an object, which makes him immortal, Harry arranges for Ron, Hermione, and Ginny-together with any of Dumbledore's Army they can summon-to keep a close watch on Malfoy and Snape. Harry also provides them with the remains of his vial of Felix Felicis, to aid them in the effort. Despite the D.A.'s watch, Malfoy provides the Death Eaters entrance into Hogwarts, and a battle ensues. Thanks to Felix Felicis, Ron, Hermione and Ginny are unharmed by the Death Eater's hexes during the battle. Snape kills Dumbledore during the battle when Malfoy proves that he is unable to. During his funeral, Ron comforts a weeping Hermione. Ron and Hermione vow to help Harry find and destroy the Horcruxes and kill Voldemort, even if it means leaving Hogwarts.

Attention is drawn several times to Ron's deepening relationships to Harry and Hermione, with unresolved romantic tension with Hermione being one of the main subplots of the novel (and, indeed, the series as a whole). Furthermore, Harry and Ron's friendship has strengthened to the point where Harry can tell Ron that his Quidditch performance is endangering his membership on the team without either character getting personal.

New Potions professor Horace Slughorn invites Hermione to join his Slug Club. She helps Ron retain his spot on the Gryffindor Quidditch team when she secretly jinxes Cormac McLaggen, causing him to miss his last save attempt during Keeper tryouts. Hermione's feelings for Ron continue to grow and she decides to make a move by inviting him to Slughorn's Christmas Party, but he romances Lavender instead in retaliation for Hermione having kissed Viktor Krum. She attempts to retaliate by dating McLaggen at the Christmas party, but her plan goes bust and she abandons him midway through the party. Ron and Hermione continually feud with each other until he suffers a bout of poisoning from tainted mead, which makes her realise that she may lose him forever and frightens her enough to reconcile with him. Following Albus Dumbledore's death, Ron and Hermione both vow to stay by Harry's side regardless of what happens. A minor subplot in the book is that Hermione and Harry form a rivalry in Potions, as Hermione is used to coming first in her subjects and is angered that Harry does better than her at a subject, especially as he follows instructions written on his text book.

In the sixth book, Harry Potter and the Half-Blood Prince, Voldemort once again declares war, and begins to rise to power once more. He murders Amelia Bones of the Department of Magical Law Enforcement, and begins to target members of the Order of the Phoenix, including Emmeline Vance. Rowling uses several chapters as exposition to establish Voldemort's backstory. In a series of flashbacks, using the pensieve as a plot device, she reveals that Voldemort is the son of the witch Merope Gaunt and muggle Tom Riddle, Sr. However, Riddle abandons Merope before their child's birth, soon after which Merope dies. Tom Riddle, Sr. never comes to find his son. After living in an orphanage, young Tom is picked up by Albus Dumbledore, who takes him to Hogwarts. Riddle is outwardly a model pupil, but is in reality a sadist who enjoys using his powers to harm and control people. He eventually murders his father and grandparents as revenge for abandoning him. The book also discusses Riddle's hatred of "Muggles" (non-magical humans), his obsession with Horcruxes, and his desire to split his soul in order to achieve immortality. J.K. Rowling stated that the fact that Voldemort was conceived under a love potion is related to his inability to understand love.

In the main plot of the book, Voldemort's next step is to engineer an assault on Hogwarts, and to attack Dumbledore himself. This is accomplished by Draco Malfoy, who arranges transportation into Hogwarts by means of a pair of Vanishing Cabinets, which bypass the extensive protective enchantments placed around the school. The cabinets allow Voldemort's Death Eaters to enter Hogwarts, where a battle commences and Dumbledore is cornered. Hogwarts professor (and triple agent) Severus Snape uses the Killing Curse against Dumbledore when Malfoy is unable to do so himself.

In Harry Potter and the Half-Blood Prince, Dumbledore fetches Harry from Privet Drive and takes him to persuade Horace Slughorn to join the Hogwarts staff - his right hand, Harry notices, is shrivelled and black. During the school year, Dumbledore meets with Harry in his office to teach him of Voldemort's past because he tells Harry that it is of immense importance. Through their lessons, they visit the thoughts of others, which contained important information about the life of Voldemort, leading to his genocidal rise to power. It is learned that Voldemort created six Horcruxes to gain immortality and that they must all be destroyed before Harry goes after the final piece of Voldemort's soul that resides in the Dark Lord's body. Harry also repeatedly warns Dumbledore in most of their lessons that another student, Draco Malfoy, is working for Voldemort. Dumbledore refuses to take any action against Draco, and instead tells Harry that he already knows more about what is happening than Harry does.

By the end of book six, Dumbledore and Harry set out to the cave where Dumbledore believed a Horcrux resides. In the cave, Dumbledore drinks a potion inside the Horcrux's container; while drinking it, he begins to scream, seemingly enduring mental torture and being weakened. After the Inferi try to seize and drag Harry into the icy depths of the lake in the cave, Dumbledore suddenly recovers and conjures a fire lasso around them. Dumbledore takes the locket within and both make their way back out of the cave and back to Hogsmeade. When they return, Madam Rosmerta informs them that the Dark Mark was conjured over the Astronomy Tower and both Harry and Dumbledore set off on Rosmerta's brooms towards the tower. In the tower, Dumbledore merely converses about Draco Malfoy's plot to kill him. Several other Death Eaters enter the tower and try to persuade Draco to kill Dumbledore. When Malfoy is unable to murder him, Snape appears and immediately performs the Killing Curse on Dumbledore.

It is revealed in the final book that Dumbledore made a terrible error by placing a cursed ring on his right hand, sometime between the fifth and sixth book, forgetting the curses that must be on the ring. The ring held the Resurrection Stone, which Dumbledore hoped to use to allow him to apologise to his sister and parents. Dumbledore called Severus Snape to help him; however, when Snape arrived and assessed the curse, all he could do was contain it. Snape told Dumbledore that he had little more than a year to live. After hearing this news, Dumbledore revealed to Snape that he knew about Voldemort's plan to have Draco Malfoy kill him. He asked Snape to use the killing curse on him when the time came because it would be less humiliating and less painful than dying of the curse that was in the ring; Dumbledore also didn't want Draco to have to kill him, saying that the boy's soul was still intact, whereas Snape was fully aware that he would be merely sparing Dumbledore the pain involved in dying from the curse in his body.

Shortly after his death, Dumbledore's portrait magically appears in the Headmaster's office. His funeral is attended by students, Hogwarts teachers and staff, members of the Ministry of Magic, ghosts, centaurs, merpeople and others who wish to pay their respects. Shrouded in purple velvet, he is entombed in a white marble sarcophagus beside the lake at Hogwarts, and it is said that he is the only headmaster to be buried on the school grounds.

In the opening chapters of Harry Potter and the Half-Blood Prince, Bellatrix Lestrange and Narcissa Malfoy visit Snape at his home in Spinner's End. Narcissa's son Draco has been given a difficult task by Voldemort, and Narcissa swears Snape to an Unbreakable Vow that he will protect Draco, help him complete Voldemort's task, and finish the task himself if Draco fails. Snape claims to have been working for Voldemort (rather than for Dumbledore) ever since Voldemort's return, and explains his actions in the previous books in that light. In addition, he points out that Dumbledore's protection has kept him out of Azkaban and free to operate on Voldemort's behalf.

At the Start-of-term feast at Hogwarts, Dumbledore announces he has finally appointed Snape as Professor of Defence Against the Dark Arts. Horace Slughorn, a retired Hogwarts teacher, replaces Snape as Potions Master. Slughorn lends Harry an old Potions textbook, in which Harry finds marginalia including a variety of hexes and jinxes seemingly invented by an unknown student, and substantial improvements to the book's standard potion-making instructions. The book is inscribed This Book is the Property of the Half-Blood Prince. The notes greatly bolster Harry's performance in Potions and he impresses Slughorn. Snape, who maintains that he "never had the impression that [he] had been able to teach Potter anything at all", is suspicious of Harry's newfound Potions success.

Later, in a fight with Draco Malfoy, Harry casts one of the Prince's spells (Sectumsempra) marked "For Enemies" and is horrified when it causes devastating wounds to Draco's face and chest. Snape rushes to the scene and heals Draco's wounds, and then interrogates Harry about the source of the spell, using Legilimency to extract the source of Harry's knowledge (the Potions textbook) from Harry's mind. As Harry refuses to hand over the Half-Blood Prince's book, Snape puts him in detention during the final Quidditch match of the year.

Returning to Hogwarts after a search for one of Voldemort's Horcruxes, Harry and Dumbledore alight on the school's astronomy tower. Gravely weakened by Voldemort's protective potion, Dumbledore asks Harry to fetch Snape. Before Harry can leave, Draco Malfoy suddenly arrives intending to carry out Voldemort's ordered assassination of Dumbledore, but cannot bring himself to commit the murder. The arrival of Death Eaters and Snape interrupt them, and Snape kills the headmaster himself. An enraged Harry (who had been paralysed by Dumbledore and witnessed the killing while under his invisibility cloak) chases Snape, Malfoy, and the Death Eaters as they flee the castle. Snape easily blocks Harry's attempts to attack him with magic and even jeeringly points out Harry's mistakes, but refuses to strike back. During the confrontation, Snape reveals himself to be the "Half-Blood Prince" (being the son of muggle Tobias Snape and pure-blood Eileen Prince). Harry is unable to stop Snape before the latter passes through the school gates and Disapparates. The full story of the relationship between Dumbledore and Snape and the real reason for the killing are not revealed until the next and last book. Rowling mentioned in an interview that at this point in the series the relationship Harry-Snape has become "as personal, if not more so, than Harry-Voldemort."