Chapter 27 - "The Lightning-Struck Tower" - In which Harry and Dumbledore return to Hogsmeade and see the Dark Mark over Hogwarts. They fly to the top of the Astronomy Tower, where Draco appears and disarms Dumbledore, but can't bring himself to kill him and Dumbledore tries to convince Draco to come to the good side. Before Malfoy can decide, though, other Death Eaters show up - including Snape, who kills Dumbledore himself, blasting the body off of the tower.

Harry helps [Dumbledore] [Apparate] back to [Hogsmeade], only to run into an alarmed [Madam Rosmerta] who tells them the [Dark Mark] has appeared over Hogwarts. Harry [Acccios] two [brooms] and he and [Dumbledore] fly toward the school.

Chapter 28 - "Flight of the Prince" - In which Snape, Malfoy, and the Death Eaters flee, and Harry gives chase. On the way, he runs through a raging battle involving the Order and tries unsuccessfully to curse Snape (who we learn definitively is the Half-Blood Prince), Hagrid's hut is set ablaze and the Death Eaters escape. Harry and Hagrid then put out the fire, see students all over the grounds, and find Dumbledore's body. Harry also discovers that the locket was not a real Horcrux, but a replacement put there by the person who took the real one: R.A.B.

With the Death Eaters gone, Harry finds Hagrid and tells him that [Dumbledore] has been killed by Snape. Together, they then make their way back toward the castle.