Eurogamer. Leviathan was first spotted in June in the Extended Cut DLC code by the BioWare Social Network.
Skordi says he will play the Reaper named Leviathan, a traitor who killed one of his own kind and has been exiled for 10 years. Leviathan controls an indoctrinated Batarian colony, where Shepard meets him and can have him join the fight against the Reapers.
BioWare is scheduled to officially announce new Mass Effect 3 DLC at San Diego Comic-Con, during its Saturday panel.