MTL - Harry Potter and the Rise of the Ordinary Person - Chapter 25

... n that corridor three times.

Then the wall opposite the tapestry cracked, and a door appeared in the gap, and Joe walked in happily.

This is the House of Requirement, the most amazing house in Hogwarts. It is sometimes absent. It can only be entered when a person really needs it. When it appears, it is always arranged exactly to the needs of the helper and is very easy to use. And because its location is described in great detail in the original book, Joe found it easily the day af ...