The Leviathan is believed to be the first monster exposed to the Apostles' energy when they came to Arad. It's stronger than any other monsters, but almost impossible to find.

That night a red moon hung in the sky, just like the day I was named Dragoon.
Maybe it was because of the moon. I usually didn't hunt unless I was tasked by the Empire, but that day I went out and hunted monsters that had nothing to do with my assignments, telling myself that I was doing it to protect peace.
After I defeated one last monster, I counted the number of monsters I hunted and felt extremely satisfied by my growing strength. The power I had absorbed from the last monster continued to spread throughout my body.
It appeared, eclipsing the red moon that lit the earth brightly.
Everything instantly became dark. I looked up and became awash in a state of overwhelming fear. Under the bright moonlight, I looked into death and wondered if I was dreaming.
What changed is that my eyes were permanently branded with the terrifying image that they beheld. My body trembled uncontrollably. My hair turned gray overnight. And the memories of its inconceivable power crept behind the fear that dominated my senses.
 

"No other monster is as strong as that. Every monster has submitted to it. The king of all beasts is there. Do you think you can take him down and become a new king? Otherwise, you'll have to give up your lance here, like I did."

On a day of the red moon - From an instructor's journal