The Dark Cloud is a level in Dark Albion that, at the time of writing this guide, is locked.
If you look, you will see numbers above it, the large number being 1000000000, and a smaller number to the left.
When the top 100 Fable Heroes players amass 1000000 gold each, the world will unlock.
To get this Achievement, when the level becomes available, you will need to activate the 'Player Heads Grow with Coin Pickup,' prior to starting the level,which can be found in Help & Options then Game Mode Options in the pause menu.
With this just play the level through and collect gold as normal, eventually your head will pop. Then complete the level and bleep - bloop.
EDIT: Thanks to Mr XBob who pointed out that the cloud is now available to all. Happy Achievement Popping!
EDIT: So what happened was somebody hacked theor total money so that the world would open. Lionhead quickly resolved this, in doing so resetting the total, they restored the total to its previous figure.
Now according to the comments, the world is, again, open.. (legitimately this time)
Sorry to update this so slowly, i have other commitments that come first.