The Leviathan

The Leviathan is a creature in the game.

Role in game
Jonah considers himself to be in Leviathan's belly and often talks to himself in monologues. He speaks of being in a physical location in the Leviathan, which implies that the Leviathan might be a metaphor for The Old City.

Most people, including Solomon, believe that Jonah is mad due to his belief in Leviathan and think that he is actually talking to himself.

Leviathan is briefly mentioned as a sub-faction in a page found in the game, but most of it is blurred out, hinting that Leviathan is in fact real.

Symbolism
In mythology, the Leviathan is a mythical sea creature which is generally associated with great sea creatures such as giant whales. In the Book of Jonah, Jonah is swallowed by a giant whale, which is referenced in a later section of the game.

In Christianity, Leviathan is described in the Book of Job and is associated with the Devil or is considered to be a demon. It's land-traveling counterpart is the Behemoth, which is one of the chapters of the game.