The Cloud is a level in 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.
Complete it then, to get your achievement.
EDIT: Thanks to Mr XBob for pointing out that the cloud is now available in Albion and Dark Albion.. It became available right after I posted the guide!
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.