Leviathan

Leviathan is a mythical creature believed to have the power to kill a Reaper and the focus of Mass Effect 3: Leviathan. During the Reaper invasion in 2186, Admiral Hackett orders Task Force Aurora to pursue myths with any relation to the Reapers, such as Leviathan, in the hopes of discovering the means to stop them. According to Dr. Garret Bryson, Leviathan was a possible apex predator that even the Reapers feared.