The word "Leviathan" appears five places in the Bible, and the Book of Job 41 is dedicated in describing Leviathan in detail:
Book of Job 3:8 "May those who curse days curse that day, those who are ready to rouse Leviathan "; NIV
Book of Job 41:1-34: "Canst thou draw out leviathan with an hook? or his tongue with a cord which thou lettest down?...He beholdeth all high things: he is a king over all the children of pride." KJV (quoted 1 and 34 only)
Psalms 74:14: "Thou brakest the heads of leviathan in pieces, and gavest him to be meat to the people inhabiting the wilderness." KJV
Psalms 104:24,25: "O LORD, how manifold are thy works! in wisdom hast thou made them all: the earth is full of thy riches. So is this great and wide sea, wherein are things creeping innumerable, both small and great beasts." KJV;
Isaiah 27:1: "In that day the Lord with his sore and great and strong sword shall punish leviathan the piercing serpent, even leviathan that crooked serpent; and he shall slay the dragon that is in the sea." KJV
The king over the children of pride is a reference to Satan (the Accuser). Just as he is referred to in the New Testament as a red dragon in Rev.12 having seven heads, so in Psalm 74:14: "Thou didst crush the heads of the Leviathan,..."