I distinctly remember Bastion having 20 Steam achievements, because it's a nice even number, and I had them all. Now, however, there are some more, a new one being Ride the Wind, with no explanation text whatsoever:
How do I unlock that? I want it.
Easy to get it seems. Just go through the first few scenes and you should get it automatically. Just don't fall off the edge in the Wharf District, because then the game will continue without awarding the achievement and you'll have to start over.
It's most likely related to either the DLC that's coming out on the 14th, or to the Steam Winter Sale. The major Steam sales usually have in-game achievements linked to entries in drawings.
Oh, and apparently at one point the description said "Ride the Skyway in The Wharf", so that may be how you get it once it's active.
It is related to Bastion free DLC / Update 08 (released on 14th December on Steam)
This patch introduced four new achievements and new modes. Regarding the specific achievement you are asking for, here it is the description.
Ride the Wind
The other three achievements are:
All in the Mind
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