Room of Requirement
The Room of Requirement is a hidden room within the Hogwarts castle, on the seventh floor in the left corridor. It is found only in the circumstance that a witch or wizard needs it, and it will change into whatever the user is in need of at the moment it is found. Neither the room itself nor the people inside it appear on the Marauder's Map.
 Harry Potter and the Order of the Phoenix
In the fifth book, Harry asks Dobby if he knows of any place within the castle of Hogwarts where he and Dumbledore's Army could safely meet without being caught. Dobby explains the Room of Requirement, or the "Come and Go Room", to Harry, how it is a room that is sometimes there and sometimes not. He goes on to say that he has used it when Winky has gotten drunk from Butterbeer to let her recover. Harry decides to use the Room of Requirement as the D.A. headquarters throughout the school year. It provides the army with several useful books on spells and charms.
In the film adaptation of the Order of the Phoenix, Neville Longbottom is the one who introduces the Room of Requirement to Harry, as Dobby was removed from the film.
 Harry Potter and the Half-Blood Prince
In Harry Potter and the Half-Blood Prince, Harry uses the Room of Requirement to hide the Half-Blood Prince's potion book, which he had secretly been using to excel in his Potions classes. It is later discovered that Draco Malfoy had been using the room all year long to learn how to bring Death Eaters into Hogwarts castle from a Vanishing Cabinet. When the room transforms into the room filled with numerous items such as the Vanishing Cabinet, it is referred to as the "Room of Hidden Things."
 Harry Potter and the Deathly Hallows
In the final installment of the series, Neville Longbottom and numerous other Hogwarts students use the Room of Requirement to hide from the Carrows, the merciless, brother-sister Death Eaters who were recently employed to teach at the school. A passage was added in order to connect to the Hog's Head (located in Hogsmeade Village) for the purpose of obtaining food, as the Room is unable to produce food by itself. Aberforth Dumbledore, owner of the Hog's Head, leads Harry, Hermione, and Ron through the passage connecting the pub and castle past a portrait of his and Albus's deceased little sister, Ariana.
Later Harry realizes that one of Lord Voldemort's horcruxes, Rowena Ravenclaw's diadem, is being contained in the Room of Hidden Things. When he, Ron, and Hermione go to the room for the diadem, they run into Malfoy, Crabbe, and Goyle. Crabbe conjures up a Fiendfyre spell, which spreads throughout the room, killing himself and causing Harry and the others to evacuate via broom.
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