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A personally-annotated copy of Advanced Potion-Making belonged to Severus Snape while he attended Hogwarts School of Witchcraft and Wizardry. Snape's nickname for himself was the "Half-Blood Prince", and so he signed this nickname along the bottom of the back cover of the book.
Advanced Potion-Making is a Potions textbook used by Hogwarts for N.E.W.T level Potions classes and authored by Libatius Borage. It contains a variety of recipes for various potions, many of which Snape improved by means of amending procedures and scribbling notes in the margins.
Under Severus Snape's possession
The first known owner of this copy of Advanced Potion-Making was Severus Snape, the future Potions Master of Hogwarts, when he was still attending as a student. Not being well off, the book was second-hand; nothing is known of its original owner.
When Snape attended Hogwarts as a student, he had acquired this textbook for N.E.W.T.-level Potions with Horace Slughorn and then signed it with his own self-proclaimed nickname and signature, the "Half-Blood Prince". Snape, being a talented wizard even at a young age, had made alterations to the many recipes within the book for even better effects. He also made notes in the margins of several spells that he had invented himself, including Sectumsempra, Levicorpus, Langlock, and Muffliato.
When Severus left Hogwarts, he fell in league with the Death Eaters, though he never lost his old copy of Advanced Potion-Making. When he eventually returned to Hogwarts to work as Potions Master, he kept this copy of Advanced Potion-Making in a cupboard in his classroom (perhaps to use its annotations for his own teaching, as it was obviously not meant to be used by students). When he changed teaching positions and Horace Slughorn gained the classroom, he left the book there.
Under Harry Potter's possession
- "On the other hand, the Prince had proved a much more effective teacher than Snape so far..."
- —Harry's thoughts towards the Half-Blood Prince, ironically unaware that it was actually Severus Snape himself[src]
About twenty or so years later, Harry Potter took a N.E.W.T. Potions class under the newly reinstated Slughorn. Harry had not bought his own textbook prior to arriving at Hogwarts, as he believed that he failed to qualify for advancement; Snape had stated that he would only accept students who had received 'Outstanding' in their O.W.L.s, but Slughorn was quite content with the 'Exceeds Expectations' that Harry had received.
As such, Slughorn loaned one of the older books left behind by previous students. Harry, by chance, received this book. While initially annoyed by the scribbles that made it difficult to read the original instructions, when Harry tried Snape's methods, he achieved better results than even Hermione Granger. When Harry got a new book from Flourish and Blotts, he then swapped the front covers and gave Slughorn the new one with a combination of the Severing and Mending Charm, while keeping this one for himself, much to Hermione Granger's displeasure.
For the rest of his sixth year at Hogwarts, Harry became better at potions than he had ever been in his time with Severus Snape and even managed to save Ronald Weasley's life with a bezoar when the latter had been poisoned. This book also helped Harry to win a bottle of Felix Felicis during his first class, which later proved essential in helping him acquire a memory from Slughorn about Tom Riddle and Horcruxes. Harry also read through the notes within out of sheer interest, and his friends said he goes as far as to sleep with it.
Over the course of the year, Harry began trying the Half-Blood Prince's self-invented spells written in the textbook. However, when he used Sectumsempra on Draco Malfoy, he was unaware of its effects and Malfoy was critically injured. Nevertheless, Harry was shocked when he saw the deadliness of the curse.
When Madam Pince saw the book with its graffiti-covered pages, she flew into a rage, despite Harry insisting that it was not the library's. She clawed the air for it, and throwing angry claims at Harry for defiling a book with writing.
During one of Professor Slughorn's Slug Club parties, Snape was told of Harry's sudden improvement in the art of potion-making. Snape then suspected that his old textbook may have fallen into Harry's hands. When Harry used Sectumsempra on Draco, Snape was able to confirm his suspicions, but was unable to get his hands on the book, as Harry had stashed the book inside the Room of Requirement and used Ron's book as a decoy. As a result, Snape (correctly) accused Harry of being a liar and cheater, and punished him with a multitude of detentions for injuring Malfoy and cheating in potions.
Hermione Granger resented the book, for it did not follow the "official instructions." She did some research on possible candidates for the Prince's identity, but the only information she found pertained to Eileen Prince, Harry stating that he was certain that the writer was male. Later, Snape revealed that he was the Half-Blood Prince, and that he owned the book before. This came as a major shock to Harry, when he learned that the Half-Blood Prince, who had taught him so much over the year, happened to be the person he hated the most (especially after Snape killed Albus Dumbledore). Ginny Weasley was also displeased when she heard Harry is following instructions from an unknown individual's book, as she had personally experienced.
It is unknown if Harry removed the book from the Room of Requirement after hiding it there, but if not, then it was most likely destroyed when Vincent Crabbe released Fiendfyre in the room during the Battle of Hogwarts.
Altered Potion formula
- Draught of Living Death
- Crush the Sopophorous bean with flat side of silver dagger, releases juice better than cutting.
- Add one clockwise stir after seven anticlockwise stirs to obtain the clear colour faster.
- Poison Antidote
- Simply shove a bezoar down the victim's throat
- Elixir to Induce Euphoria
- Heavily corrected
- Add a sprig of peppermint to counter excessive singing and nose-tweaking side-effects.
- Levicorpus ("n-vbl")
- Liberacorpus (counterspell for Levicorpus)
- Sectumsempra ("for enemies")
- Toenail Growth Hex
Behind the scenes
- In the film adaptation of Harry Potter and the Half-Blood Prince:
- The only spell amongst those noted within the book to appear was Sectumsempra.
- Harry and Ron saw the battered book and a better condition one in the classroom cupboard and they fought over the better one, with Ron winning it, leaving Harry with the Half-Blood Prince's old copy.
- After Harry inflicted Sectumsempra on Malfoy and fled, he had Ginny Weasley hide the book for him within the Room of Requirements, so he would not be tempted into looking for it again.
- Levicorpus appears in the film of Harry Potter and the Order of the Phoenix during one of the D.A. meetings, used on Nigel Wolpert - nearly a year before any of the DA leaders should have known about it. Its incantation was also wrongly spoken aloud.
- The film prop used for this book appears to have its text comprised of excerpts taken from the Wikipedia article on alchemy, as can be seen on this version of the page from around the time of the film's release.
- The book can be compared to T. M. Riddle's Diary. Both were used by their respective owners as an outlet to use Dark Magic: Tom Riddle used his diary to create his first Horcrux, while Snape used his textbook to create original magic. This further highlights the personal similarities between Riddle and Snape. Whether this is intentional on Rowling's part or merely a coincidence is unknown. Ginny Weasley was vocal when she learned of Harry's inclination to obey this textbook's extra instructions, as she herself experienced negatively from Riddle's diary.
- Harry Potter and the Half-Blood Prince (First appearance)
- Harry Potter and the Half-Blood Prince (film)
- Harry Potter and the Half-Blood Prince (video game)
- Harry Potter and the Deathly Hallows (Indirectly mentioned only)
- Harry Potter and the Deathly Hallows: Part 2 (Appears in flashback(s))