As Harry is once again relating his adventures in Dumbledore’s office, he reports that Lord Voldemort has risen again and that Voldemort gave a very insightful monologue to his Death Eaters. Dumbledore later tells Harry his thought process at the moment.
“[Voldemort] made a most illuminating and alarming statement to his Death Eaters. ‘I, who have gone further than anybody along the path that leads to immortality.’ That was what you told me he said. ‘Further than anybody.’ And I thought I knew what that meant, though the Death Eaters did not. He was referring to his Horcruxes, Horcruxes in the plural, Harry, which I do not believe any other wizard has ever had. Yet it fitted: Lord Voldemort has seemed to grow less human with the passing years, and the transformation he has undergone seemed to me to be only explicable if his soul was mutilated beyond the realms of what we might call ‘usual evil’ . . .” (HBP501-2)
An interesting point to consider is that this passage seems to indicate that Dumbledore used Legilimency on Harry as Harry related the events in the graveyard. It is wholly implausible that a severely traumatized Harry would be able to recite Voldemort’s speech word-for-word after the night he’d just had—after all, he’s not Hermione! It’s curious to consider how many times Dumbledore performed Legilimency on Harry over the years, but we now have two instances that are almost certain.
Anyway, now Dumbledore’s two-year-old theory is confirmed: that Voldemort did indeed make multiple Horcruxes. And now Dumbledore has to do research! What are the other Horcruxes? Dumbledore already has some solid candidates for what might be a Horcrux—Slytherin’s locket, the Peverell ring, and Hufflepuff’s Cup.27 With the diary, that’s already four.
But Dumbledore has one huge unanswered question here: how many Horcruxes are there total? Would Voldemort have dared to make four Horcruxes? Or did he make even more than that? This is the most important piece of information that Dumbledore needs. After all, wouldn’t it be awful if Dumbledore destroyed ten Horcruxes and attempted to kill Voldemort, only to find out that Voldemort had made an even dozen? Or inversely, what if Dumbledore wasted time hunting for a sixth and seventh Horcrux if Voldemort only made five, and innocent people died in the meantime? Dumbledore needed to know exactly how many Horcruxes there were! And in the meantime, he could start researching what and where the Horcruxes might be.
Of course, this is all easier said than done. How could he find out how many Horcruxes Voldemort made? Short of asking Voldemort himself, there seems to be no possible way. Unless. . . what if, when Tom Riddle was learning about Horcruxes, he decided on a certain number? Eventually, this line of thinking would lead Dumbledore straight to Slughorn.
I believe Dumbledore got the altered memory from Slughorn that very summer in 1995. Upon seeing that Slughorn had made it useless by excluding the only information Dumbledore needed—the planned number of Horcruxes—Dumbledore attempted to lure Slughorn to a job at Hogwarts.
Didn’t it ever strike anyone as odd that Dumbledore was completely unable to find a D.A.D.A. professor in 1995? Sure, the applicant pool is rather thin after a four-decade-long curse. But we’ve seen that Dumbledore has no objection to hiring substandard professors when he needs them there (see: Hagrid, Trelawney, etc.). And keeping Umbridge out of Hogwarts would seem like a pretty big priority, so why wouldn’t Dumbledore just ask Kingsley or someone to fill in for a year?
Because Dumbledore was keeping a position open for Slughorn, that’s why. He wanted Slughorn to come back to Hogwarts, where Dumbledore could work on extracting that much-needed memory from him. And then Snape would be put in the D.A.D.A. position, because Snape is still needed at Hogwarts as well. Dumbledore was so desperate to get Slughorn that he did not even make a back-up plan, which is how Umbridge got foisted onto Hogwarts at the very last second.
The Scar
When Dumbledore received confirmation of his theory about Horcruxes, this allowed him to make another important deduction: the likelihood of Harry’s scar containing a bit of Voldemort’s soul. When I first wrote about Dumbledore and Horcruxes, many of my readers took issue with the theory that Dumbledore did not know everything about the Scarcrux prior to Order of the Phoenix, based on the conversation he had with Harry at the end of Chamber of Secrets.
“Unless I'm much mistaken, he transferred some of his own powers to you the night he gave you that scar. Not something he intended to do, I'm sure. . .”
"Voldemort put a bit of himself in me?" Harry said, thunderstruck.
"It certainly seems so." (CS333)
The interpretation here hinges on whether Dumbledore means “a bit of himself” literally—I believe he does not. He merely refers to the transfer of powers as a bit of Voldemort, instead of referring to a bit of soul. We must keep in mind that Harry and Voldemort are delving into unknown realms of magic. Dumbledore did not even know that Voldemort had split his soul until the end of Chamber of Secrets. He would not have made the leap that Voldemort’s soul was so unstable as to have bits flying off it, not until he received confirmation at the end of Goblet of Fire about Voldemort’s multiple
