Dumbledore's Army
During "Deathly Hallows", Aberforth reveals a secret passage that leads into Hogwarts' Room of Requirement, where Dumbledore's Army has set up headquarters. The Hog's Head entrance is hidden behind a portrait of Ariana Dumbledore, the younger sister of Albus and Aberforth. Before the Battle of Hogwarts begins, the passage is used to evacuate underage students from the school. The remaining members of Dumbledore's Army and the Order of the Phoenix gather at the Hog's Head before entering the castle through this passage to fight Voldemort and his Death Eaters.