The Leviathan lives in the deepest part of the ocean. It has been sleeping there for a millenia. So long, that it has been covered by sediment, and other lifeforms have taken up residence on his surface, unaware that he is NOT the ocean floor.
Once, the world was covered in water, and the Leviathan could feed on the lesser beings at will. Leviathan feeds on souls as much as meat. So just eating unintelligent sealife is not enough to sustain him.
Leviathan has not been idle. In his dreams, he has been working. Long ago, he saw what he needed in the depths of space. He has drawn a massive body of ice from elsewhere in the star system to this world. He has carefully set about changing its motion and trajectory to bring it into close orbit with this world. And thus begins his second stage.
The astronomers have long known about The Wanderer, a stellar body passing through the system. Records are incomplete to know that it was once a frozen moon of a gas giant. By the time observers had the technology to see it, it appeared to be a comet or some such.
In recent days, The Wanderer has grown in size and everyone has noticed it. It's tail has formed a rainbow like stream, that initially trailed away behind it. Now, the tail forms an arc that bends back to this world in a rainbow display.
As days progress, it becomes more obvious that The Wanderer has entered orbit over this world as a new moon. There is a continual rainbow streaming off of it, visible even in day time.
It has become increasingly rainy, as the sun's rays from the moon are melting it and the planet's gravity is drawing it in. Since the planet is in rotation, the rain hits in waves. It turns out that it is effectively raining everyday, somewhere on the planet, by way of this melting moon. As the planet rotates and the moon orbits, the rain simply moves, it doesn't stop unless the planet is between the moon and the sun.
With all the moisture, the people have begun to get sick, and get pneumonia. Sores appear on both sides of their neck. The sickness passes, leaving a thin line of a scar on both sides of their neck.
As months pass, the ocean levels rise, flooding of coastal cities begins. It may be noticed now, that no one who has had the Sickness drowns. The scars on their neck, it turns out, have become Gills.
A year later, the new moon has shrunk greatly in size. All but the highest points on the planet are flooded. Society has adapted, because everyone who had the Sickness has discovered they can live underwater. So, they simply rebuild their lives, in their underwater cities.
The Leviathan awakes. It is time to feed.