"Leviathan," The first piece of Mass Effect 3 single-player DLC since "Extended Cut," will look into the history of The Reapers and explain how they came to be. According to EA, it will feature "gripping and emotional storytelling, compelling new characters, powerful weapons and unique upgrades."
Here's the full, official description from EA's Summer Showcase:
"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 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."
More info as it becomes available.