As Olin mentions in the tweet, you can pre-purchase the DLC right now through Steam. Though you don't strictly need to secure your copy of a digital-only product ahead of time, since there's no concern about scarcity, you do save $1.50 by doing so.
Posted on Dec 11th 2013 5:00PM
Posted on Dec 11th 2013 12:45PM
Posted on Dec 11th 2013 9:34AM