Voice actor Anthony Skordi has perhaps naughtily told Eurogamer website that he is in fact voicing Leviathan for the Mass Effect 3 DLC.
Leviathan is apparently a Reaper gone rouge who can prove to be of a most valuable War Asset to Shepard and the Alliance forces as part of the ME3 campaign.
It's been suggested that early reports indicated that the "DLC will feature Shepard being tasked with rescuing a scientist from a Reaper-indoctrinated mining facility that has been under the control of a Reaper named "Leviathan" for a decade. Leviathan has been in exile since killing one of his own, providing Shepard with an opportunity to turn it against the other Reapers."
The DLC would not mark Leviathan's first entry into the Mass Effect series. Scanning a planet in the original Mass Effect reveals "The Leviathan of Dis" which in the context of that game was intimated at being downed space ship. Mass Effect 3 mixes that up by having the Batarian character Balak confirm that the ship is actually Leviathan and that the Batarians have reactivated it incidentally....