The extensions pack for Marathon, Marathon Rubicon will be available shortly for download.
Four years in the making, the game completes Bungie Software's Marathon trilogy, which involves players taking on the role of a lone security officer, battling to survive and triumph over starships crewed by savage aliens.
Rubicon is the largest Marathon project ever, its new content including an extra 360 new textures, 12 characters, different sceneries and eight weapons.
Texture artist and map-maker for Bungie's original Marathon Infinity, Randy Reddig, said: "The visuals are the nicest I've ever seen come out of Marathon. If you've played any Marathon game, Rubicon will re-ignite your love for the series."
Plotlines can played a number of different ways and feature various endings.