The following post will contain mild spoilers about the plot of the "Leviathan" downloadable content pack for Mass Effect 3. If you don't want to know anything about the storyline, then you should stop reading, and simply know that a voice actor in the new content confirmed the news toEurogamer, and that a release date is currently unknown.
If you want to know more, then we can tell you (seriously, stop reading if you don't want to know anything) that the pack will star Leviathan, a reaper who killed one of his own kind ten years ago and has been in exile. He controls an indoctrinated Batarian mining facility, and may be recruited in the fight against the reapers.
A partial script of the DLC was found inside the code of the Epilogue DLC. More information on the DLC is due out next week at Bioware's official panel at San Diego Comic Con. It's speculated that Leviathan's plot may reveal further information about the Reapers.