You have to have beaten the game to get this key. After you beat it, return to the City of Aurora on your Ruler of Albion anniversary. Find the tattoist and talk to to the man next to him. You need to purchase the Ancient Key (4,000 gold). Once you have it, head to the northwest corner of the Shifting Sands area to unlock a previously locked door (duh). You are now in the Crossroads of Passing.