The Leviathan is a mythical sea monster or dragon that is mentioned in Job 41.
Leviathan in the Bible
Job 41 describes the Leviathan as a fire-breathing sea monster or dragon. "Smoke pours from his nostrils" and his breath is so hot that it "sets coals ablaze" with the "flames [that] dart from his mouth." According to Job, the Leviathan is so massive that it causes the waves of the sea.
1 Can you pull in the leviathan with a fishhook or tie down his tongue with a rope?...
9 Any hope of subduing him is false; the mere sight of him is overpowering...
14 Who dares open the doors of his mouth,ringed about with his fearsome teeth?...
15 His back has rows of shields tightly sealed together;
16 each is so close to the next that no air can pass between...
18 His snorting throws out flashes of light; his eyes are like the rays of dawn.
19 Firebrands stream from his mouth; sparks of fire shoot out.
20 Smoke pours from his nostrils as from a boiling pot over a fire of reeds.
21 His breath sets coals ablaze,and flames dart from his mouth.
31 He makes the depths churn like a boiling caldron and stirs up the sea like a pot of ointment.
32 Behind him he leaves a glistening wake; one would think the deep had white hair.
Origin of the Leviathan
Some scholars believe that the Leviathan is based upon similar legends that belonged to ancient peoples the Jews came into contact with. For instance, the Canaanite sea monster Lotan or the Babylonian sea goddess Tiamat.
The Leviathan in Jewish Legend
Just as the Behemoth is an unconquerable monster of the land and the Ziz a giant of the air, the Leviathan is said to be a promordial sea monster that can't be defeated. Job 26 and 29 say that "the sword...has no effect" and that "he laughs at the rattling of the lance." According to legend, the Leviathan will be an entree served at the messianic banquet in Olam Ha Ba (the World to Come). In this instance, Olam Ha-Ba is conceived of as a Kingdom of God that will exist after the Messiah comes. Talmud Baba Batra 75b states that the archangels Michael and Gabriel will be the ones who slay the Leviathan. Other legends say God will slay the beast, while yet another version of the story says that the Behemoth and the Leviathan will fight a mortal battle at the end of time before being served at the banquet.
Sources: Talmud Baba Batra, Book of Job and "The Encyclopedia of Jewish Myth, Magic and Mysticism" by Rabbi Geoffrey W. Dennis.