The Key to a Greater Key is a side quest in Fable III that becomes available once you have defeated the Darkness.
Walkthrough[]
After defeating the Darkness, speak to Vallo near the Tattooist in the City of Aurora, he will offer to sell you a key for 4,000 gold, saying it will lead to another greater key and that in turn will unlock a great treasure. Purchase the Ancient Key and head out into the Shifting Sands.
From the City of Aurora head out towards the Shifting Sands (through the outpost if you built one as part of your judgements). After passing under the great stone arch, head straight across the desert until you come to an opening with stairs leading down to a door (this is to the left of the exit from Shadelight as you look towards it). It unlocks with the Ancient Key and allows you to enter the Crossroads of Passing.
Once you reach the other side of the chasm inside, you will reach your reward, a Gold Key.
See Crossroads of Passing for more details.
Description[]
An Auroran businessman has an interesting proposition for you.
Conclusion[]
Well, you've bought an ancient key, and it does have an inscription on it. But is it real? Does it lead to a 'greater treasure'? Only one way to find out.