Why is Thomas Hobbes's book called "Leviathan?"

The inspiration for the title of Thomas Hobbes's study of absolute monarchy came from the Old Testament. The Leviathan is a sea monster referenced in the Tanakh, also known as the Mikra or the Hebrew Bible, a collection of canonical Jewish texts.
The Leviathan is mentioned six times in Job 41:1-34. In this biblical book, the Leviathan is described as something that must be tamed:

Can you pull in the leviathan with a fishhook or tie down his tongue with a rope?
Can you put a cord through his nose or pierce his jaw with a hook?

The Leviathan can also be regarded as the whale that swallowed Jonah. Hobbes takes another quote from the Book of Job to contextualize his analysis of absolute monarchy: 

There is no power on earth to be compared to him (Job 41:24).

Looking at the frontispiece for the book, this strength is made clear with an image of a giant king rising from a landscape with a sword in one hand and a staff in the other. He hovers over the kingdom, which is in the foreground, but looks out beyond it. He has the power, it seems, to protect the land or to destroy it. Hobbes argued that only absolute monarchy -- the only valid social contract, in his view -- could protect people from succumbing to a brute state of nature. 
 
https://www.biblegateway.com/passage/?search=Job%2041&version=NIVUK