As the students of Hogwarts prepare to fight or flee, Voldemort’s voice echoes through the school, promising to leave Hogwarts untouched if Harry Potter is handed over by midnight. Pansy Parkinson of Slytherin House shouts that they should grab Harry, but the Gryffindors, Hufflepuffs, and Ravenclaws all stand in his defense. The Slytherins all leave the school, but McGonagall has to force out the members of other houses who are underage but want to stay and fight for Harry.
The professors go to man their battle positions, as Harry turns back to his search for the lost diadem of Ravenclaw. Remembering that the diadem has not been seen “in living memory,” he decides to ask the dead—the Gray Lady, who is the House ghost of Ravenclaw. Harry finds the Gray Lady, who refuses to help him until he confronts her with the threat to Hogwarts. Then she admits that she is the ghost of Helena Ravenclaw, the daughter of the founder. Helena had stolen the diadem, hoping to make herself smarter and more important than her mother. She fled to hide in a forest in Albania, and her mother, Rowena, concealed the theft. On her deathbed, Rowena wanted to see her daughter one last time, so she sent a man who loved Helena to seek her out. Helena refused to come, and the young man killed her in anger, then killed himself. The young man became the Hogwarts ghost known as the Bloody Baron, while the diadem was left in the forest.
Pressed by Harry, the Gray Lady admits that she told her story to Tom Riddle (Voldemort’s name when he was a student at Hogwarts). Harry guesses that Tom Riddle went and found the diadem in the forest, but when he made it a Horcrux, re-hid it at Hogwarts. (Harry knows from his vision that it’s at Hogwarts now.) He reasons that Voldemort’s only chance to hide it after he graduated from Hogwarts would have been the day he came to ask Dumbledore for a job, on the way to or from Dumbledore’s office.
Harry leaves Ravenclaw Tower and runs into Hagrid, who is with his giant dog Fang and giant half-brother, Grawpy. Harry leads them off in search of Ron and Hermione, seeing signs everywhere that the battle for Hogwarts has begun. As they run through the school, Harry remembers with a shock that he has seen the diadem in the Room of Requirement, in its form of “the room where everything is hidden,” when he hid his own Potions book there in Harry Potter and the Half-Blood Prince.
Finally, Harry encounters Ron and Hermione, who have gone to the Chamber of Secrets to retrieve basilisk teeth to destroy Horcruxes with. Ron was able to imitate Harry speaking Parseltongue to gain entry to the room, and they destroyed the cup. Harry, Ron, and Hermione go to the Room of Requirement, taking stock of the ever-worsening battle swirling around them. In the room, amid the labyrinth of hidden objects, Harry finds the diadem but is confronted by Draco Malfoy and his cronies, Crabbe, and Goyle, who followed Harry in to capture him, hoping to hand him over to Voldemort.
Crabbe and Goyle try to kill Harry rather than capture him, for once ignoring Draco’s leadership, and in the struggle the diadem is dropped. Crabbe uses a fire curse to try to kill Harry and his friends, and it rages out of control. Harry and Ron seize broomsticks and save Hermione, Goyle, Malfoy, and the diadem, but Crabbe dies. Outside the Room of Requirement, they see that the diadem Horcrux has been destroyed by the fire curse.