In the sixth book, Dumbledore fetches Harry from Privet Drive and takes him to persuade Horace Slughorn to rejoin the Hogwarts staff. Harry notices that Dumbledore's right hand is shrivelled and black. During the school year, Dumbledore teaches Harry of events in Voldemort's past that he feels are of immense importance. Using the Pensieve, they visit the memories of others, which contain important information about Voldemort's life and his genocidal rise to power. While using the pensieve, Harry sees a vision of Dumbledore's first encounter with the young Tom Riddle; Dumbledore had known from the beginning that the boy was dangerous, but believed that Hogwarts would change him. It is learned that Voldemort created six Horcruxes to gain immortality and that they must all be destroyed before Harry goes after the final piece of Voldemort's soul in the Dark Lord's body. Harry also repeatedly warns Dumbledore in most of their lessons that school bully Draco Malfoy is working for Voldemort. Dumbledore refuses to take any action against Draco, and instead tells Harry that he already knows more about what is happening than Harry does. By the end of the book, Dumbledore and Harry set out to the cave where Dumbledore believes a Horcrux resides. In the cave, Dumbledore drinks a potion inside the Horcrux's container; while drinking it, he begins to scream, seemingly enduring mental torture and being weakened. Dumbledore begins to call out for water after he finishes the potion, and Harry, realising he has no other choice, dips the goblet into the lake to give him a drink. When he does this though, all the Inferi that reside in the lake grab at Harry and attempt to drag him down and drown him in the lake. Dumbledore suddenly recovers, thanks to the water, and conjures a fire lasso around them. Dumbledore takes the horcrux (a locket) and both make their way back out of the cave and back to Hogsmeade. When they return, Madam Rosmerta informs them that the Dark Mark was conjured over the Astronomy Tower. In the tower, Dumbledore enjoins Harry not to interfere in the events that are about to take place there, and places him in a body-binding curse under his invisibility cloak. Hidden, Harry is unable to intervene as Dumbledore (now extremely weak) is disarmed by Draco. Dumbledore is conversing with Draco about the plot to kill him, when several other Death Eaters enter the tower and try to persuade Draco to kill Dumbledore.