Location from harry-potter-deathly-hallows by
The hidden room that becomes what you need -- it housed Dumbledore's Army as a resistance base, connected to the Hog's Head for supplies, hid Ravenclaw's diadem for decades, and burned in Fiendfyre.
The Room of Requirement served as the resistance headquarters during the Carrow regime. Neville and the DA lived here when the dormitories became too dangerous. A passage through Ariana Dumbledore's portrait connected to the Hog's Head, where Aberforth supplied food and smuggled people in and out. During the Battle, Harry entered the Room of Hidden Things to find Ravenclaw's diadem. Draco, Crabbe, and Goyle followed. Crabbe cast Fiendfyre -- cursed fire that cannot be controlled -- and the room became an inferno. Harry rescued Draco (and Goyle). Crabbe died in his own fire. The diadem was destroyed, though not by design. The room was gutted by the Fiendfyre and may never function again.
Changes based on need. As the DA hideout: hammocks, Dumbledore's Army banners, a tunnel to the Hog's Head, Ariana Dumbledore's portrait serving as a door. As the Room of Hidden Things: a cathedral-sized labyrinth of centuries of junk where the diadem Horcrux sat unnoticed. After the fire: gutted, ruined, the ceiling collapsed.
Also known as: the Room of Requirement, Room of Requirement, the Come and Go Room, Room of Hidden Things