Harry, Ron, and Hermione appear in Hogsmeade, but their appearance triggers a magical alarm that sounds like loud screaming. A dozen Death Eaters burst out of the Three Broomsticks pub in search of them. Though they remain under the Invisibility Cloak, they have nowhere to hide, and they infer from the Death Eaters’ comments that enchantments will keep them from teleporting away again. The Death Eaters unleash dementors in search of them, and Harry summons his Patronus, potentially giving their position away.
Before the Death Eaters can find them, however, a door opens in a house on the street, and a rough voice summons them inside and into a room above the Hog’s Head Inn. Still cloaked, they look out the window down at the street below, where the man who saved them—whom Harry recognizes as the Hog’s Head barman—argues with the Death Eaters. The man claims that it was he who set off the alarm, letting his cat out after curfew. He claims that the Patronus was his own goat Patronus, not Harry’s stag, and points out that Voldemort won’t want to be summoned over a cat. Mindful that the Hog’s Head bar is a convenient place for them to trade black market goods, the Death Eaters leave him alone.
Harry recognizes the man’s blue eyes as those he’s been seeing through the magic mirror, and he realizes that this man must be Aberforth, Dumbledore’s brother, and that Aberforth was the one who sent Dobby. Aberforth acknowledges that he’s been trying to keep an eye on Harry, though it was not he who led them to the sword.
Aberforth tries to convince Harry that Voldemort has already won, and that Harry should abandon his quest—whatever it is—and leave the country, before he meets Dumbledore’s fate. He reminds them of his brother Dumbledore’s penchant for lies and secrecy, and says that many of those Dumbledore loved and cared for turned out to be worse off than if he’d left them alone.
Hermione guesses that Aberforth is talking about his sister, Ariana, and prods him into giving them the real story of what happened to her. Ariana was not a Squib, as Rita Skeeter claimed. When she was six years old, as her magic was beginning to manifest itself but before she could control it, she was observed doing magic by three much older Muggle boys, who attacked her in some unspecified way, leaving her permanently unhinged. Dumbledore’s father was imprisoned in Azkaban for attacking these boys, and Dumbledore’s early flirtation with the idea of wizards dominating Muggles stemmed from anger at what had happened to his sister and father, coupled with a wish to create a world in which his sister would not have to hide.
Dumbledore returned home when his mother, Kendra, died and took responsibility for Ariana. He met Grindelwald, and the two began hatching grand plans to change the world, wanting to set off as soon as possible. Aberforth confronted them, pointing out that Ariana was in no fit state to travel or be left alone, so they had no way to do whatever it was they wanted to do. As the argument grew heated, Aberforth drew his wand, and Grindelwald used the Cruciatus (torturing) curse on him. As the three fought, Ariana came to intervene, and one of the curses the three wizards were hurling at each other killed her. Grindelwald left immediately, and Dumbledore was free to embark on his career.