A crafty BioWare has snuck titbits of data about new single-player DLC in the 1.85gb of Mass Effect 3: Extended Cut files.
Watch out for spoilers!
"So a Reaper goes rogue and kills one of its own. That's what we're looking at here, right?" says one of the lines in this hidden script.
The new mission has Shepard sent to rescue scientist Ann Brynson from a indoctrinated Batarian mining colony. Brynson can live or die in the content, and Leviathan can be convinced to join your side - likely becoming a war asset in the process.
Leviathan has been mentioned in the Mass Effect series before, as scannable object Leviathan of Dis in the original game, and referenced again in Mass Effect 3 by Balak, who says it's a dormant Reaper that the Batarians accidentally activated.