Tapestry

   You were walking down the seventh floor corridor, when you noticed a peculiar tapestry depicting someone teaching trolls in pink tutus to dance. Little did you know, this was that tapestry of Barnabas the Barmy and his infamous attempt to teach ballet to trolls. You pass in front of the tapestry a few times, and suddenly you realize it has disappeared, and there's a door in its place. Congratulations! You have found The Room of Requirement