In Book 6 we discover some big secrets, the biggest of which might just be that of the Horcrux. A Horcrux is a kind of magic so dark and so horrible that books about it have been removed from the Hogwarts library. A Horcrux is a kind of magic by which one can make oneself immortal by ripping one's soul into pieces and by encasing these pieces in objects. The only way one can rip one's soul into pieces is to kill another person.
Rowling uses the idea of the Horcrux to show just how horrible murder is – not only for the victim, but also the murderer. Killing someone tears the murderer's soul. We learn that Voldemort has torn his soul into seven pieces and created six Horcruxes. Through the Horcruxes we also see that Voldemort is incredibly evil. He has taken magic to a place it has probably never gone before, by making not one, but six Horcruxes. This is unheard of, and proves that he is far worse than any Dark wizard that has come before him.
Through his carefully hidden Horcruxes, Voldemort is able to flee death and to become immortal. The problem is that the more he rips his soul apart, the weaker and more decrepit his soul becomes – which explains the inhuman nature of Voldemort. However, since Voldemort doesn't value the soul, he doesn't realize that, in a way, he is weakening himself. This is another way in which Voldemort is very different from Dumbledore and Harry, who value the soul and the ability to love.
Dumbledore's is in the process of discovering and destroying the six Horcruxes in Book 6. He believes that Tom Riddle's diary, which possessed Ginny Weasley in Harry Potter and the Chamber of Secrets, was once a Horcrux. He has also finds and destroys the Gaunt family ring, which was also a Horcrux. Harry and Dumbledore capture the third Horcrux, the Slytherin locket, when the venture to the cave of Voldemort's childhood, but Harry later discovers that the locket is a fake, and the real Horcrux has already been stolen by someone with the initials R.A.B. That leaves three more Horcruxes and the last shard of soul which lies in Voldemort himself.
Dumbledore and Harry speculate that the three Horcruxes might be in the form of Hogwarts heirlooms, such as something owned by Helga Hufflepuff or Rowena Ravenclaw. Nagini, Voldemort's snake, may also be a Horcrux. But by the end of Book 6, all Harry has is speculation and guesswork on his side – he will have to figure out where Voldemort might have hidden his remaining four Horcruxes and what form these Horcruxes might be in. Voldemort likes to collect items with historical weight and sentimental value, trophies of his horrific acts and crimes, and so it is imperative that Harry gather all of the knowledge he has of Voldemort and his past.
Schools are places of safety and learning. They are a harbor for young minds away from the troubling world. However, in Harry Potter 6 we watch Hogwarts turned upside down and inside out with tragedy, worry, and danger. A sign of the times, Harry and his friends must learn to adjust to a word in which there is no safe place. Voldemort is gaining so much momentum and force that even Dumbledore's enchantments cannot protect the Hogwarts community.
Interestingly, we come to understand through Book 6 just how powerful Hogwarts is. It is so powerful that Voldemort twice tries to become a teacher there. We all know that knowledge is power, and Hogwarts is a vessel of hundreds and hundreds of years of study, discovery, exploration, and fine-tuning. Some of the best wizards and witches of the world have attended Hogwarts. Voldemort would have been all too pleased to advance his own campaign from the comfy chair of a Hogwarts classroom.
The Half-Blood Prince's Potions Textbook
The Half-Blood Prince arms Harry with powerful tips, tools, and ideas that allow Harry to both coast in Potions class and to learn powerful magic. The irony here is that the Half-Blood Prince, we come to find at the end of the story, is most likely Professor Snape himself. The very magic that Harry soaks up and appreciates so dearly comes from the man who kills Dumbledore. Is the textbook planted in Slughorn's classroom? If not, why didn't Snape hold onto it? What exactly does the book teach Harry to do?
Upon Dumbledore's death, Fawkes the phoenix sings and then flies away for good. As you know, a phoenix is a symbol of rebirth – it dies by bursting into flames, but is then reborn from the ashes. Dumbledore was all about giving people second chances – he was ready to give Voldemort a second chance when he returned to Hogwarts ten years after graduating, asking for a teaching job. And he certainly put his trust in Snape when others would not. The Phoenix is a powerful force in the world of Harry Potter for its reminder of the circle of life. Death is a necessary part of life. Even magic cannot combat death. This information stands in strong relief to Voldemort's efforts to conquer death.
Still, we are filled a sudden sadness and emptiness when Fawkes leaves Hogwarts for good after appropriately mourning Dumbledore's death. The silence that follows is a heavy one, and we are left to wonder what comes next once Dumbledore is gone. Who else is there who believes so fully in the good of people, who believes in giving people a second chance? There's a finality to Fawkes's departure that suggests something more than Dumbledore's life has come to an end.
You might want to think about where else we see the phoenix in the Harry Potter series. What other symbolic weight might Fawkes have?
The potion's greatest power is in showing us that our innate abilities are as powerful as magic. When Harry tricks Ron by pretending to pour the potion into his pumpkin juice before a Quidditch game, Ron believes that nothing can go wrong because he is under the influence of a potion. As a result, he plays better than he has ever played before and he wins the game for Gryffindor. When Harry reveals that no luck potion was ever involved, Ron realizes that he was capable of greatness all along. These wizards and witches of Hogwarts must learn both how to use magic and how to get along without it by trusting themselves. Magic is often not the most powerful tool in the shed.
Interestingly, the good luck potion also helps Harry secure the much-needed Horcrux memory from Slughorn. The fact that Harry is only able to retrieve this memory via magic reveals the extent to which humans will go to prevent others from discovering secrets, from having their real selves revealed. Slughorn's self-hatred is so huge that he will stop at nothing to make sure his memory is kept secret. In this case, magic helps undo the damage of human guilt.