Harry, Ron, and Hermione receive a tear-stained letter from Hagrid announcing Aragog’s death and requesting their presence at his funeral. They decide it’s too dangerous to travel outside of Hogwarts at night. Ron tells Harry that he should try using Felix Felicis to acquire Slughorn’s memory, and Hermione agrees. Ron and Hermione head off to Hogsmeade for their Apparition tests. Hermione passes her exam, but Ron doesn’t. After dinner, Harry takes a gulp of Felix Felicis, and a warm glow fills him. Harry puts on his Invisibility Cloak, telling his friends he’s decided to go to Hagrid’s. Harry heads toward Hagrid’s, running into Slughorn on the way. Harry explains about Aragog and Hagrid. Slughorn tells Harry that the venom of an acromantula is extremely rare and almost impossible to gather. Harry invites Slughorn to the funeral. Harry, Hagrid, and Slughorn step into Hagrid’s backyard, where a grave has been dug. While Hagrid is crying, Slughorn approaches the giant spider and collects a bottleful of venom. After Aragog is buried, they sit down for drinks.
Harry senses that he should abstain from drinking too much, and nonverbally casts a Refilling Charm, making sure Slughorn and Hagrid’s glasses stay full. Hagrid and Slughorn continue to drink heavily. Harry tells Slughorn about the pain of his mother and father dying so young and reminds Slughorn of how much he liked Harry’s mother, Lily. Harry tells a very drunk Slughorn that the only way to stop Voldemort is to give Harry the true memory. Slughorn tells Harry he is not proud of what it contains, but he hands it over anyway. Harry feels the Felix Felicis wearing off and walks back to the castle. Dumbledore is delighted with Harry’s success and empties Slughorn’s memory into the Pensieve. This time, there is no fog or loud voices. When Voldemort asks about Horcruxes, Slughorn explains that a Horcrux is an object in which a person has concealed part of their soul. Even if the person is attacked or destroyed, a small part of his soul lives on outside of his body—and therefore he can never die. Slughorn goes on to explain that very few would want that kind of existence, and that death is often preferable. Splitting one’s soul is an unnatural violation and can only occur when one commits the supreme act of evil, murder.
Slughorn tells Voldemort not to repeat anything he has just heard, and the memory ends. Dumbledore tells Harry that he suspects Voldemort has split his soul into seven pieces, and that no wizard has ever attempted multiple Horcruxes. The only way to kill Voldemort is to destroy all seven Horcruxes. Dumbledore believes that the seventh Horcrux is Voldemort’s regenerated body and should be destroyed last. Of the other six, two are already gone—the diary Harry obliterated his first year at Hogwarts, and Marvolo’s ring, which Dumbledore withered his hand destroying. Harry suggests Slytherin’s locket and Hufflepuff’s cup. Dumbledore wonders if one of the remaining two Horcruxes belonged to Ravenclaw, since Gryffindor’s only known relic, the sword, is safe in Dumbledore’s office. Dumbledore believes that Nagini, the snake, is the sixth. It is revealed that when Dumbledore leaves Hogwarts for long stretches of time, he is out hunting for Horcruxes. Dumbledore thinks he is close to finding another, and tells Harry that he can come along to help destroy it. Dumbledore reminds Harry that his ability to love makes him a great and powerful wizard, explaining that this capacity for love is the only thing that has kept him from being lured to the Dark Arts.
Chapters 22 and 23 contain some of the most significant moments in the Harry Potter and the Half-Blood Prince, as Harry and Dumbledore finally unlock the secret to defeating Voldemort. After weeks of putting it off, Harry finally manages to coerce the necessary memory out of Slughorn. Harry uses a combination of alcohol and guilt, invoking his mother’s memory and playing on Slughorn’s admitted fondness for her, miraculously managing to get Slughorn to give up the true memory. Ironically, Harry does this by using the Felix Felicis potion Slughorn himself made and gave to Harry as a reward. Even though Slughorn realizes that the memory will reveal what Slughorn actually told Voldemort about Horcruxes and will destroy his reputation in Dumbledore’s eyes, he seems to understand that Voldemort needs to be stopped and that Harry is the person destined to do it. Slughorn finally risks being cast in an unfavorable light to help Harry and Dumbledore. He makes a sacrifice for the greater good, no matter how much he wishes he didn’t have to reveal his true memory of that very unfortunate night. It is clear from Slughorn’s tone that although he is willing to tell Voldemort what a Horcrux is, he does not encourage any wizard to ever get involved with something so dark and terrible.
Once Harry has secured the memory, he and Voldemort are able to learn exactly what Voldemort knew about Horcruxes. By murdering seven people and tearing his soul into seven separate pieces, each of which has been magically contained within an object, Voldemort has become as close to immortal as any man or wizard can be. The only way to truly destroy Voldemort is to get rid of all seven Horcruxes, including the final Horcrux, which lies inside Voldemort’s regenerated body. Finally, Harry understands what Dumbledore has been doing when he leaves Hogwarts and feels a restored faith in his headmaster. Rather than leaving the school unguarded and ripe for attack, Dumbledore has been searching for the keys to Voldemort’s destruction. Harry cannot think of a more worthy use of his headmaster’s time and, consequently, is relieved to know that Dumbledore is not letting down his beloved Hogwarts. Dumbledore believes that the remaining four Horcruxes are all hidden in objects that are significant to Voldemort, and once again we are reminded of Voldemort’s penchant for stolen or important objects. This is clearly the climax of Harry’s private lessons with Dumbledore, and the final piece of information that Dumbledore feels he needs to bestow upon Harry to prepare him for his ultimate battle.
Even though Voldemort is an extraordinarily powerful wizard, gripped by a desire to kill Harry Potter as soon as possible, Dumbledore insists on making sure Harry understands that he is actually the stronger of the two wizards. Harry’s whole and untarnished soul is far more powerful than Voldemort’s dark, divided soul, and that is the power the prophecy refers to when it says that Harry has a “power the Dark Lord knows not.” Harry still finds this difficult to believe. Although he has heard Dumbledore speak of his ability to love time and time again, he still cannot fathom how something so simple and instinctual could ever help him destroy an evil as great as Voldemort’s. But Dumbledore knows that even if the prophecy did not exist, Harry would still feel compelled to destroy Voldemort, not only for all of the evil he has inflicted upon the Muggle and Wizarding worlds, but because he murdered both of Harry’s parents.