Leviathan
Leviathan is a sea serpent demon noted in theology and mythology. It is referenced in several books of the Bible, including Psalms, the Book of Job, the Book of Isaiah, the Book of Amos, and, according to some translations, in the Book of Jonah; it is also mentioned in the Book of Enoch.
Leviathan is often an embodiment of chaos and threatening to eat the damned after life. Christian theologians identified Leviathan with the demon of the deadly sin envy.
Origin
Leviathan specifically is mentioned six times in the Old Testament, in Job 3:8, Job 40:15–41:26, Psalm 74:14, Psalm 104:26 and twice in Isaiah 27:1.
The element of competition between Yahweh and the sea monster and the use of Leviathan to describe the powerful enemies of Israel may reflect the influence of the Mesopotamian and Canaanite legends or the contest in Egyptian mythology between Apep, a snake that embodies chaos, and the sun god Ra. Alternatively, the removal of such competition may have reflected an attempt to naturalize Leviathan in a process that demoted it from deity to demon to monster.
Christian/Hebrew interpretation
When the Jewish midrash (explanations of the Tanakh) were being composed, it was held that God originally produced a male and a female leviathan, but lest in multiplying the species should destroy the world, he slew the female, reserving her flesh for the banquet that will be given to the righteous on the advent of the Messiah. A similar description appears in Book of Enoch (60:24), which describes how the Behemoth and Leviathan will be prepared as part of an eschatological meal.
Leviathan can also be used as an image of the Devil, endangering both God's creatures—by attempting to eat them—and God's creation—by threatening it with upheaval in the waters of Chaos. A "Dragon," being the usual translation for the Leviathan in the Septuaginta, appears in the Book of Revelation.
St. Thomas Aquinas described Leviathan as the demon of envy, first in punishing the corresponding sinners (Expositio super Iob ad litteram).
Church of Satan interpretation
Anton LaVey in The Satanic Bible has Leviathan representing the element of water and the direction of west, listing it as one of the Four Crown Princes of Hell. This association was inspired by the demonic hierarchy from The Book of the Sacred Magic of Abramelin the Mage.
The Church of Satan uses the Hebrew letters at each of the points of the Sigil of Baphomet to represent Leviathan. Starting from the lowest point of the pentagram, and reading counter-clockwise, the word reads "לויתן": (Nun, Tav, Yod, Vav, Lamed) Hebrew for "Leviathan."
Other sources
Leviathan was one of the signatories of the diabolical pact with French priest Urbain Grandier.