Once Dumbledore dies, the Death Eaters flee, and Harry is released from the spell Dumbledore cast on him.
Harry tears down the Tower stairs, chasing after Snape and Malfoy, but encounters a raging battle in the Hogwarts halls between the Order of the Phoenix and the Death Eaters. Everyone from McGonagall to Neville seems to be fighting. Blood is everywhere, but no one seems to be terribly hurt.
The Death Eaters continue to flee, and Harry chases after Snape, who deflects every kind of magic Harry throws at him.
Outside on the Hogwarts grounds, Snape prevents the Death Eaters from harming Harry, reminding them that Voldemort commanded that Harry be left alone for him to deal with at a later date.
Angry, Harry tries the Prince's Sectumsempra spell on Snape.
Snape mocks Harry for using the very spells and magic that he himself invented, confirming that he is, in fact, the Half-Blood Prince.
Before Snape leaves, he sets Hagrid's house on fire.
Harry helps Hagrid put out the fire, and he tells Hagrid that Dumbledore has died.
They rush to the place where Dumbledore's body has fallen.
Everyone is overcome with grief.
Harry discovers that the Horcrux locket has fallen out of Dumbledore's pocket. After examining the locket, he realizes that it is not the right one. It does not have a Slytherin S on the outside and it does not look like the locket Hepzibah Smith showed Voldemort all of those years before.
Inside of the locket is a note to Voldemort from a person whose initials are R.A.B. R.A.B. tells Voldemort that he has destroyed the Horcrux: "I face death in the hopes that when you meet your match, you will be mortal once more" (28.201).
Harry realizes that Dumbledore weakened himself for no good reason – the Horcrux was a fake.