Dhruin
SasqWatch
Joystiq has a description of the upcoming Leviathan DLC for Mass Effect 3. It's only a single paragraph on an EA fact sheet but it's the only detail officially released so far:
More information.Mass Effect 3: Leviathan is the first story-driven, single player DLC to be made available since Mass Effect 3 Extended Cut. Leviathan expands upon the events of Mass Effect 3 with gripping and emotional storytelling, compelling new characters, powerful weapons and unique upgrades. Players will be able to discover more about the origins of the reapers as they race across the galaxy to unravel the mystery that surrounds the fabled Leviathan.