Text files and a plot summary for Mass Effect 3's forthcoming Omega DLC pack have been discovered (or so fans claim) in Mass Effect 3 itself. Perhaps the "D" should stand for "disc-based"? Stop reading now if spoilers make you a sad space panda.
The "downloadable" mission takes place aboard Omega Station, Mass Effect 2's foremost den of scum and villainy. Overlord Aria T'Loak has been ousted by the Illusive Man and Cerberus, and needs Shepard's help to retake the facility. Along the way, you'll meet and possibly kill Mass Effect 2's Zaeed Massani, who has signed up with Cerberus.
Complete the mission to unlock a supply of Element Zero for the war effort. Here's the plot summary with more specifics.
Aria tells you that Omega is now under a blockade from Cerberus. Since a direct attack didn't work, Cerberus is now trying to starve Omega into submission by blockading supply ships coming in through the regular Omega relay. The level's appearance will change to reflect this - the Afterlife has gone quiet, the music is off, the flashing lights are gone, maybe a soup kitchen has taken over the dance floor. Aria requests that you steal into the blockade and board the command ship. Take down the blockade so Omega can be resupplied.
As punishment for breaking the neutrality pact, Cerberus launches an attack against Omega. The enemy is attempting to destroy Omega's shields, which will allow asteroids and meteors to smash into the station. You must repel the assault and ensure the shield remain operational.
BioWare announced Omega alongside Mass Effect 4 and a new next gen IP last week. The studio also has Dragon Age 3 on the boil for late 2013. Busy times.