Mass Effect 3's Citadel DLC add-on is now yours for the taking on Xbox Live Marketplace. It comes in two downloads, weighing in at just over 2GB in total, and costs 1200 MP. Queue up the file here.
The pack takes place aboard the Citadel, as you may quite possibly have already deduced - check out BioWare's guide to accessing the new Mass Effect 3 content in-game. "Commander Shepard and the Normandy crew get a much deserved break, but their shore leave is short lived," reads the blurb. "Face down a new villain before catching up with old friends from Mass Effect 1 and 2."
Screenshots and a fuller description of the plot can be found here, together with a few closing BioWare remarks on the trilogy at large. Are you getting involved? Trailer is below.