Horace Slughorn
Horace E. F. Slughorn is a character in the Harry Potter series of novels written by J.K. Rowling. He is a teacher at Hogwarts School of Witchcraft and Wizardry and makes his first appearance in the sixth book of the series, Harry Potter and the Half-Blood Prince. He is also a good and trusted friend of Albus Dumbledore.
Prior to the beginning of the series, Horace Slughorn was the long-serving Potions master at Hogwarts and Head of Slytherin House since at least the 1940s. He retired in the 1980s, but returned to both positions, as Potions master at the beginning of Harry's sixth year at Hogwarts and as Slytherin Housemaster at the end of that year.
During his long initial period of employment at Hogwarts, Slughorn taught several of the main characters in the Harry Potter story. Among his pupils were the young Severus Snape, and Lily Evans, Harry's mother and one of Slughorn's favourite students, whom he repeatedly praises as "one of the brightest I ever taught...vivacious...charming...cheeky". Significantly, however, an earlier cohort of students included Tom Marvolo Riddle. On one occasion, Riddle questioned Slughorn about Horcruxes, artifacts which could act as vessels for pieces of a person's soul in order to grant immortality. Having learnt of the nature of Horcruxes, Riddle went on to perform the necessary Dark magic to create his own Horcruxes, which contributed to his formidable power in his later reign of terror under his assumed name of Lord Voldemort.
At some point after his retirement and the resurgence of Voldemort described in the novels, Slughorn went into hiding, concealing all knowledge of his whereabouts from both sides in the growing conflict in the Wizarding world. However, by the beginning of the sixth book in the series, Hogwarts headmaster Professor Albus Dumbledore has agreed to move Professor Severus Snape, erstwhile Potions master, to the post of Defence Against the Dark Arts teacher, and he manages to locate Slughorn and engage him to succeed Snape.
At the beginning of Harry Potter and the Half-Blood Prince, Slughorn returns to his previous position as Potions teacher. He is initially unwilling to return to the school as he has been in hiding from Voldemort, and fakes a disappearance to mislead Dumbledore. He is persuaded when he sees Harry, who has accompanied Dumbledore, and remarks on Harry's likeness to his parents. Slughorn is reminded that, while on the run, he has not been able to take advantage of the very numerous personal and social contacts he has collected through his life thus far. Dumbledore also makes a point of allowing Slughorn to see that he is now wearing Voldemort's ring.
Slughorn demands less stringent entry criteria for Advanced Potions than his predecessor, Professor Snape, lowering the required grade from a near-perfect O (Outstanding) to an above-average E (Exceeds Expectations). This last-minute change enables Harry to take Potions to NEWT level, but because he had not expected to be allowed to join the course he has none of the necessary materials. Slughorn lends him an old textbook until he can procure his own.
In his first Potions class, Slughorn offers a small amount of Felix Felicis potion, known as "liquid luck", to the student who brews the best cauldron of the Draught of Living Death. He tells the class that he has taken Felix Felicis twice, once at age 24 and once at age 57. It will give the user constant good fortune for a day, but must be used sparingly. Harry wins the potion, with the help of handwritten notes in the borrowed textbook, which is also signed 'property of the Half-Blood Prince'. Harry continues to use the book in classes with great success, causing Slughorn to remark repeatedly that Harry has inherited his mother's abilities at potions.
Dumbledore, who has undertaken to teach Harry about his nemesis Voldemort by the use of a Pensieve or receptacle for memories, has at some point obtained from Slughorn a memory of Riddle's question about Horcruxes. However, when viewed, the memory turns out to be incomplete. Slughorn, ashamed to admit his involvement in telling Riddle about the Horcruxes, has deliberately obscured parts of the memory - specifically, his responses to Riddle's questions.
Dumbledore commissions Harry to obtain the full memory from Slughorn. Partly because Harry's eyes remind him of Lily's, partly because Harry has taken a little of the Felix Felicis, and partly because he has been drinking, Slughorn is eventually persuaded to divulge to Harry the unexpurgated memory. Viewed, the memory convinces Dumbledore that his theory that Voldemort has employed six Horcruxes to achieve immortality is correct.
Towards the end of the story, after the death of Dumbledore and the flight of Snape, Slughorn again represents Slytherin house on the orders of Professor McGonagall, now Acting Headmistress. He advises the new headteacher that Hogwarts should remain open. Readers await the final book in the series to discover whether Slughorn decides to stay at Hogwarts or return to obscurity.