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.
Atop the Astronomy Tower
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.
Harry Versus Snape
Harry chases Snape across the lawn as the Death Eaters launch curses at him and at Hagrid. Snape admits he's the Half-Blood Prince, though he won't let Harry use an [Unforgivable Curse], and won't let the other Death Eaters kill Harry, either - and soon they all get past the gates and [Disapparate].
In the Hospital Wing