Harry tells Aberforth that Dumbledore was never free of his past, and describes how Dumbledore, when he drank a potion and went out of his mind in the previous book, was begging an unseen figure to hurt him instead of “them”—clearly a memory of seeing Grindelwald hurting Aberforth and Ariana. Harry says that he hasn’t given up on the Order of the Phoenix and intends to defeat Voldemort, and Aberforth agrees to help him get into Hogwarts.
Aberforth turns to an oil painting of Ariana on the wall and tells the picture of Ariana that she knows what to do. Ariana turns around within the picture and walks down a tunnel, growing smaller and smaller until she disappears, eventually returning through the picture with a bedraggled and scarred Neville Longbottom, who emerges from the painting into the room.
Neville wants to know if rumors he’s heard about Harry breaking into Gringotts are true, and Harry confirms this. Neville describes what’s happening at Hogwarts: the Carrow siblings, Amycus and Alecto, have been made professors and put in charge of discipline. Amycus teaches students how to use the Cruciatus Curse, while Alecto teaches Muggle studies with an anti-Muggle bias.
Neville proudly shows the scars he earned for standing up to the Carrows. Neville remembered that when Harry stood up to unjust authority figures, it gave the rest of the students hope, so he tried to fill this role after Harry left school. Luna was taken away from school, and Ginny never returned after Easter break, so Neville found himself increasingly on his own in carrying out underground acts of resistance against the new regime. Eventually, the Death Eaters tried to stop Neville by going after his grandmother, who put a Death Eater in the hospital and then went on the run. Neville knew that it was time for him to disappear, and he has been in hiding in the Room of Requirement, which they had used in Harry Potter and the Order of the Phoenix.
Neville leads Harry and company through the portrait into the Room of Requirement. About twenty students are hiding in it, all members of Dumbledore’s Army and supporters of Harry who’ve been driven into hiding. They are wild with joy at seeing Harry and his friends. Soon after, Luna Lovegood and Dean arrive. All of the students are eager to help Harry, and are unhappy and resentful to hear that Harry, Ron, and Hermione are on a mission alone that they won’t disclose or accept help with. With some prodding from Ron and Hermione, Harry realizes that he doesn’t have to be quite as cryptic as Dumbledore and can recruit help in searching for the Horcrux without explaining what it is.
Harry tells the assembled students that they’re looking for a distinctive object, and that they don’t know what it is but think it might be associated with Ravenclaw (given that other objects were associated with other houses). Luna volunteers that there is a legend about a lost diadem of Ravenclaw, which has been gone for centuries. Harry could see a reproduction of it on the statue of Rowena Ravenclaw in the Ravenclaw common room.