You can pre-purchase now via Steam or from SEGA Digital Store though you can probably get some better deals from various online Steam key re-sellers. Just remember that many Steam games and their downloadable contents are region restricted so you may want to check from where you purchase them to avoid the keys you have purchased from becoming just a digital trash.
This is the first mini campaign (and maybe the last) DLC for Attila featuring the campaign of Flavius Belisarius, a Roman general who served under the Emperor Justinian on his ambitious military campaigns to reconquer the lost Mediterranean territory of the former Western Roman Empire. My guess the DLC will feature much of Italy and western Europe since the historical Belisarius early campaigns was against Vandals in the North Africa and then the Ostrogothic Kingdom in Italy.
Creative Assembly has also released a new Rally Point which also includes interviews with developers and most importantly for me is this announcement below.
"In this new Rally Point, we announce a brand new Total War Campaign Pack! Also we have an ace interview with Mike Simpson, Creative Director on Total War, about the original Medieval and Shogun games, which are going to be heading to Steam. "