First you have to complete "The Key to a Greater Key", thanks to which you will gain the key opening the gate in the north-west corner of the Shifting Sands. To reach it, start at the City of Aurora and head towards the stone arch marked on the map . Pass under it and keep moving forward. The gate is at the end of the path .
Inside the ruins you will eventually reach platforms with blue arrows . Stepping onto the proper one will cause another square to appear, but choosing the wrong one will summon a group of monsters. The proper path is as follows: straight, right, left, right, straight .
This way you will reach a room with a button mounted in the floor . Step onto it and return to the first platform. Now you need to follow this pattern: right, forward, right, forward. Once you use the second switch, more stairs will appear and the gate on the other side of the room will open .
Behind it is the gold key  .