Room of Requirement

The Room of Requirement is a hidden room in Hogwarts castle that exists only for someone who needs it, taking whatever form the need demands. Harry Potter and the Goblet of Fire mentions it first in passing, when Albus Dumbledore recalls discovering a room full of chamber pots at an urgent moment and never finding it again, and Harry Potter and the Order of the Phoenix presents the room properly, along with its rules. The seeker must walk past the right stretch of blank seventh-floor wall three times, concentrating on what is needed; a door then appears onto a room equipped precisely for the purpose, whether that means bookshelves, cushions, or something stranger. The castle's house-elves know it as the Come and Go Room, and say it opens only to real need.