Mass Effect 3 is an action role playing game developed by BioWare and published by Electronic Arts for Microsoft Windows, PlayStation 3 and Xbox 360.
Mass Effect 3: Leviathan is single-player DLC that was announced at EA's Summer Showcase.  It was first teased at San Diego Comic-Con 2012.  Playing the content will change the ending of Mass Effect 3. 
Leviathan includes new systems, new areas on the Citadel, brand new characters, and exclusive weapons: the AT-12 Raider Shotgun and M-55 Argus Assault Rifle. Lock word document for editing. Leviathan will be available on PC, Xbox 360 and Playstation 3 for $10 (800 Microsoft points). It was released on August 28, 2012 for all platforms, with a delayed PS3 release in Europe set for August 29, 2012.
Something lurks in the dark corners of space, something powerful enough to kill a Reaper. Shepard must discover the most closely guarded secret in the galaxy before the Reapers silence it forever. Discover more about the origins of the Reapers as you race across the galaxy to find the Leviathan. Unravel the dark history of the Reaper Race before it is too late.
Taking place during the events of Mass Effect 3, players will be thrust into the darkest corners of space where they will hunt a mysterious being rumored to be powerful enough to destroy a Reaper. As they race against time, they will begin to unravel the secret of the Leviathan.
- New locations
- New Characters
- Bryson's Husk Head experiment (Cabin item)
- 11 War Assets
- 5 Weapon Modifications
Other Appearances of Leviathan
Download high school of the dead. Leviathan is a name fans might recognize. The Reaper has been mentioned before by returning character Balak, a Batarian who first appeared in the original Mass Effect's Bring Down the Sky DLC and later made a brief appearance in Mass Effect 3. According to Balak, Leviathan indoctrinated many inhabitants of the Dis system after being unintentionally activated.
Files included in the Extended Cut leaked that new story content would focus on a rogue Reaper named Leviathan. The files included several lines of dialogue that hinted at missions revolving around a search for the Reaper.  Ahead of its official announcement, the content was confirmed by actor Anthony Skordi, who provides the voice of Leviathan in the DLC. 
A dump of the files uploaded to PasteBin included a character asking “So a Reaper goes rogue and kills one of its own. That's what we're looking at here, right?” Characters also seem to be on the search for Leviathan as he takes control of various people, commenting “No sign of Leviathan at these coordinates” and “That mining facility.. everyone there [is] under Leviathan's control.”