Kreacher's Cave

Kreacher's Cave does, as the title suggests, belong to Kreacher, but isn't actually a cave. He has cleared out an old, abandoned classroom, and is beginning to fill it with the junk he has “borrowed” from teachers and students behind their back, rather in the manner of a magpie. It's dusty and dark, with one window that lets in a few bleak shafts of light. The only thing that was originally inside it that he wasn’t cleared out is a strange mirror, where when you look inside it, you don’t see a reflection of any sort. Carved on the top in ornate, curling script is a single word. Desire.

Kreacher himself is an apparently insane - but harmless - old house-elf. He doesn't welcome visitors, but he doesn't shoo them out, either. However, if you even try to steal or take any of your stuff back, he'll be on you in a flash. Welcome to the Cave.