Severus Snape's copy of Advanced Potion-Making

"This Book is the Property of the Half-Blood Prince"

- Severus Snape's signature in the book

Severus Snape, also known as the Half-Blood Prince, owned a personally-annotated copy of the book Advanced Potion-Making. At a young age, Snape was shown to be very talented in potions, and he made many alterations to the recipes and instructions in the book for better effects. In addition to these alterations, Snape also invented a number of spells, including Sectumsempra, Levicorpus, and Muffliato which he wrote down in the margins of the book.

Under Severus Snape's possession
When Snape attended Hogwarts, he acquired this textbook for his sixth year Potions class with Horace Slughorn, and signed it with his nickname, the "Half-Blood Prince." Snape, being a brilliant wizard even at a young age, had made alterations to the recipes within the book for better effects. He also made notes of spells he invented. When Snape graduated from school, he left the book in the potions classroom.

Under Harry Potter's possession
About twenty years later, when Harry Potter took N.E.W.T. Potions under the newly reinstated Slughorn. Harry did not buy his own textbook because he thought he failed Snape's expectations for advancement. As such, Slughorn loaned one of the older books left behind. Harry, by chance, got a hold of this book. When Harry got a new book from Flourish and Blotts, he swapped the front covers and gave Slughorn the new one, while keeping this one for himself, much to Hermione Granger's displeasure.

For the rest of the school year, Harry has became a much better potioneer than he had in his time with Snape and even learned to save Ron Weasley's life with a bezoar when he became poisoned. This book also helped Harry to win a bottle of Felix Felicis during his first class, which became essential in helping him acquire a memory from Slughorn about Tom Riddle and Horcruxes.

Over the year, Harry tried the Half-Blood Prince's self-invented spells written in the textbook, often with hilarious results. 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.

During one of 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 Malfoy, 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.

Hermione Granger resented the book, for it did not follow the "official instructions." She did some research, but the only information she found pertained to Eileen Prince. 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).

It is unknown if Harry removed the book from the room after, 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 the silver knife releases more juice.
 * Add one clockwise stir after seven counter-clockwise 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.

Invented Spells

 * Langlock
 * Levicorpus (non-verbal)
 * Liberacorpus (counter-spell for Levicorpus)
 * Muffliato
 * Sectumsempra (for enemies)
 * Toenail Growth Hex

Behind the Scenes

 * Of all of Snape's spells found in the Potions book, only Sectumsempra appears in the film version of Harry Potter and the Half-Blood Prince.

Appearances

 * Harry Potter and the Half-Blood Prince
 * Harry Potter and the Half-Blood Prince (film)
 * Harry Potter and the Half-Blood Prince (video game)