To access this room, you must first collect all eight gems from the treasure chests located around the Chalice of the Gods dungeon. Once you have the gems, you'll find the gems guardian in the room that's fourth from the top in the last column on the map. You need to take this route to finish the game anyway.
Once you're in the room, simply walk past the guardian on the sign with the 8 on it. The achievement will unlock as soon as you walk into the next room.