Until now only multiplayer content has been added with new maps and challenges. This new addition lets fans "really explore, discover and learn more" about the Mass Effect universe.
Aside from new story and action there are also two new additions to the armoury; AT-12 Raider shotgun and M-55 Argus assault rifle. It adds more end-game dialog.
"Discover more about the origins of the Reapers as you race across the galaxy to find the Leviathan. Travel to new systems, wield the AT-12 Raider Shotgun and M-55 Argus Assault Rifle, and unravel the dark history of the Reaper Race before it is too late," blurb BioWare.
We also get more of the Citadel to explore apparently. Leviathan is available to buy through Origin on PC for $10/£6. It's also available through PlayStation Network and Xbox Live Marketplace.