Leviathan is a boss in Final Fantasy IV: The After Years who appears on the moon. He fights as he did in Final Fantasy IV. If the player hopes to regain him as a summon Rydia should be in the party and kept alive for the duration of the fight. Leviathan is considered an aquatic enemy and Thunder-based attacks deal high damage to him.
Leviathan (Biblical Hebrew לִוְיָתָן or לווייתן Liwyāthān) is a violent sea creature referred to in the Hebrew Bible as well as the Talmud and some other ancient Jewish texts. During the golden age of seafaring, the term Leviathan became synonymous with any large sea creature, sometimes describing whales. Bible scholars interpreted Leviathan to be either a demonic sea creature and an enemy of God.
The Talmud and other ancient Jewish sources refer to Leviathan more specifically as a sea serpent, sometimes with multiple heads capable of breathing fire, that was made on the fifth day of Creation. It is said that during the end of days, God will make tents, or sukkah, out of the skin of Leviathan for the righteous to live in. Some have speculated that Leviathan was based on a real animal, the most popular theory being the Kronosaurus and the Nile crocodile.
Being mentioned in the Final Fantasy series as the King of the Seas, its incarnation also appears to derive from the legends of the Shinto god Ryūjin or Owatatsumi, said to be the dragon god that reigns over the seas. This is further reinforced by the parallels of the Leviathan faith of Wutai in Final Fantasy VII.
In some cultures it is believed Leviathan lives at the bottom of Mariana Trench.