The Room of Requirement

• Type: Magical Room
• Location: Seventh floor of Hogwarts Castle, opposite a tapestry depicting Barnabas the Barmy's foolish attempt to teach trolls ballet.
• Owner/Residents: The room is an intrinsic part of Hogwarts Castle itself and has no owner. It has been used by countless students and staff over the centuries, including Albus Dumbledore, Tom Riddle, Harry Potter, Dumbledore's Army, Draco Malfoy, and members of the Hogwarts resistance during the Second Wizarding War.
• Key Features: The Room of Requirement is Unplottable and does not appear on the Marauder's Map. It only becomes accessible when a person has a genuine need for it. The room's interior and contents transform to perfectly match the seeker's requirements.

The Room of Requirement, also known to the house-elves as the Come and Go Room, is a secret room within Hogwarts whose existence is known only to a select few. To make the room appear, a person must walk past the section of blank wall on the seventh floor three times while concentrating hard on what they need. A door will then materialise. The room's nature is to provide whatever the user requires, with some limitations. It cannot, for instance, create food, as this is one of the five Principal Exceptions to Gamp's Law of Elemental Transfiguration. However, it can provide access to food, as demonstrated when it created a secret passage to the Hog's Head Inn. The magic of the room is highly advanced and seems to be sentient, able to interpret the user's needs with remarkable precision. One of the room's most significant and stable configurations is the Room of Hidden Things. In this form, it appears as a vast, cathedral-sized chamber cluttered with an immense collection of objects hidden by generations of Hogwarts inhabitants. This “room full of junk” contains everything from broken furniture and old potion bottles to forbidden magical artifacts. It was in this form that Tom Riddle hid Rowena Ravenclaw's Diadem, and where Harry Potter later hid his copy of the Advanced Potion-Making textbook. The contents of this version of the room appear to be constant unless an item is physically removed or destroyed. Its existence was discovered by various individuals over time. Albus Dumbledore stumbled upon it by chance when he was in urgent need of a lavatory. Fred and George Weasley used it as a hiding place from Argus Filch, and Dobby the house-elf knew of it and told Harry Potter how to find it.

The Room of Requirement plays a pivotal role in the second half of the series:

• In Harry Potter and the Order of the Phoenix, it serves as the secret headquarters and training ground for Dumbledore's Army. When Harry needed a secure location to teach practical Defence Against the Dark Arts, the room provided a large space complete with cushions for duelling practice, Dark Detectors like Sneakoscopes and Foe-Glasses, and a library of defensive spellbooks. The D.A. used it until their discovery by Dolores Umbridge, facilitated by Marietta Edgecombe's betrayal.
• In Harry Potter and the Half-Blood Prince, the room becomes central to Draco Malfoy's mission for Lord Voldemort. He uses the Room of Hidden Things to store and repair a broken Vanishing Cabinet, which he eventually links to its twin at Borgin and Burkes, allowing Death Eaters to infiltrate Hogwarts. During this time, Draco's powerful need to hide his activities effectively locked others, including Harry, out of the room.
• In Harry Potter and the Deathly Hallows, the room becomes a sanctuary for students rebelling against the regime of the Carrows and Headmaster Severus Snape. Led by Neville Longbottom, the room provided a safe haven with hammocks, washing facilities, and a secret passage to the Hog's Head Inn in Hogsmeade. During the Battle of Hogwarts, this passage allowed reinforcements from the Order of the Phoenix and Dumbledore's Army to enter the castle. The room was last seen in its “Room of Hidden Things” configuration, where Harry, Ron Weasley, and Hermione Granger searched for Rowena Ravenclaw's Diadem. In the ensuing conflict, Vincent Crabbe unleashed a cursed Fiendfyre spell that raged out of control, destroying the diadem Horcrux and seemingly the Room of Requirement itself. It is unknown if the room's powerful magic could ever repair such damage.

In the film adaptations, the entrance to the Room of Requirement is depicted as a complex, mechanical wall of shifting gears and stone, a visual element not described in the novels (film). During the Battle of Hogwarts sequence in the final film, the character of Gregory Goyle casts the Fiendfyre that destroys the room, a change from the book where Vincent Crabbe is responsible. This change was made because the actor playing Crabbe was unavailable for filming (film).