Leviathan of Dis

The Leviathan of Dis is the name given to a gigantic corpse that disappeared from a crater on Jartar in 2163. It is believed to be the remains of a genetically engineered starship and its age was originally placed at nearly a billion years old.

Unfortunately, there was little time to study and uncover the true nature of this mysterious find. Not long after its discovery by a batarian survey team, a batarian dreadnought visited the Dis System, after which the Leviathan mysteriously disappeared. The batarians have since strenuously denied that the Leviathan existed at all. They become even more vocal when shown documentation of the Leviathan by salarian researchers before its disappearance.

Provided he survived the events of Bring Down the Sky, Balak will tell Shepard that the Leviathan was actually an inactive Reaper which ended up indoctrinating most of the system when the scientists accidentally reactivated it. Afterwards they infiltrated and sabotaged the Hegemony's defenses opening the way for the Reaper invasion of their territory.

Trivia

 * A leviathan generally refers to any being of gargantuan proportions.