Note: Despite being published by Vertigo comics, the Unwritten is a creator owned property and is not canonically part of the wider DC universe.

Who is the unwritten Leviathan?

The Leviathan is a cosmic entity from the comicbook series The Unwritten. It either is or contains the collective consciousness of everyone that experiences stories, and it forms a symbiotic relationship with the worlds; it eats stories and in turn these stories are sustained.

Is Unwritten Leviathan Canon?

It's definitely canon to DC.