The World Key is, as of January 2014, a useless accessory item.
How to Obtain
World Keys can be looted from a Mechanical Chest or traded with other players.
The World Key has no uses as of November 2014.
This item was added in 2013.
In a test for new player retention, world keys did have a use. New players were given small "home worlds" that were mini private worlds. In order to make these worlds open to the public, a world key needed to be used. However, the test was unsuccessful, and along with home worlds, the function of the world key is gone.