After collecting at least 8 Keys (Either 8 in Arena 1 or 7 in Arena 1 and the 1 in Potionology) you will unlock this room.
From the entrance and looking at the fountain, go right into the next hall and take a left once at the fork. You will see a door to your left with a pink book symbol on it, stand in front of it and press
to open it.
Once inside, Jump down and head into the back of the room. You will find a golden lever hidden behind a curtain in the window. Stand in front of it and press
to activate it and make a platform move. Turn around and head up the stairs on the other side of the room, then take a left at the top. Follow the path past the door you came in and up another set of stairs. Once at the top of these stairs, turn around and jump onto the platform when it comes down to you, then jump into the little alcove with the treasure chest.
On the other side of the alcove you will see a small rail with blue gems on it. Jump onto the rial and carefully walk along it to find another golden lever at the end of it, so stand in front of it and press
to pull it. Doing so will raise a pillar in the back of the room near the first golden lever.
Head down to the back of the room (no fall damage so just jump off the rail) and fall down into the hole to find the Key. Just grab it to unlock this Achievement.