Sea serpent in the context of "Vritra"

Sea monsters are beings from folklore believed to dwell in the sea and are often imagined to be of immense size. Marine monsters can take many forms, including sea dragons, sea serpents, or tentacled beasts. They can be slimy and scaly and are often pictured threatening ships or spouting jets of water. The definition of a "monster" is subjective; further, some sea monsters may have been based on scientifically accepted creatures, such as whales and types of giant and colossal squid.

Leviathan (/lɪˈvaɪ.əθən/ le-VIE-ə-thən; Hebrew: לִוְיָתָן, romanized: Livyāṯān; Greek: Λεβιάθαν) is a sea serpent demon noted in theology and mythology. It is referenced in the Hebrew Bible, as a metaphor for a powerful enemy, notably Babylon. It is referred to in Psalms, the Book of Job, the Book of Isaiah, and the pseudepigraphical Book of Enoch. Leviathan is often an embodiment of chaos, threatening to eat the damned when their lives are over. In the end, it is annihilated. Christian theologians identified Leviathan with the demon of the deadly sin envy. According to Ophite Diagrams, Leviathan encapsulates the space of the material world.

In Gnosis, it encompasses the world like a sphere and incorporates the souls of those who are too attached to material things, so they cannot reach the realm of God's fullness beyond, from which all good emanates. In Hobbes, Leviathan becomes a metaphor for the omnipotence of the state, which maintains itself by educating children in its favour, generation after generation. This idea of eternal power that 'feeds' on its constantly self-produced citizens is based on a concept of conditioning that imprints the human's conscience in a mechanical manner. It deals in a good and evil dualism: a speculative natural law according to which man should behave towards man like a ravenous wolf, and the pedagogically transmitted laws of the state as Leviathan, whose justification for existence is seen in containing such frightening conditions.

Worm, wurm or wyrm (Old English: wyrm; Old Norse: ormr; Old High German: wurm), meaning serpent, are archaic terms for dragons (Old English: draca; Old Norse: dreki/*draki; Old High German: trahho) in the wider Germanic mythology and folklore, in which they are often portrayed as large venomous snakes and hoarders of gold. Especially in later tales, however, they share many common features with other dragons in European mythology, such as having wings.

Prominent worms attested in medieval Germanic works include the dragon that killed Beowulf, the central dragon in the Völsung Cycle – Fáfnir, Nidhogg (Old Norse: Níðhǫggr), and the great world serpent, Jǫrmungandr, including subcategories such as lindworms and sea serpents.

The giant oarfish (Regalecus glesne) is a species of oarfish of the family Regalecidae. It is an oceanodromous species with a worldwide distribution, excluding polar regions. Other common names include Pacific oarfish, king of herrings, ribbonfish, and streamer fish.

R. glesne is the world's longest ray-finned fish. Its shape is ribbon-like, narrow laterally, with a dorsal fin along its entire length, stubby pectoral fins, and long, oar-shaped pelvic fins, from which its common name is derived. Its coloration is silver and blue with spots of dark pigmentation, and its fins are crimson. Its physical characteristics and undulating mode of swimming have led to speculation that it might be the source of many "sea serpent" sightings.

In Norse mythology, Jörmungandr (Old Norse: Jǫrmungandr, lit. 'the Vast 'gand'', see Etymology), also known as the Midgard Serpent or World Serpent (Old Norse: Miðgarðsormr, "worm of Midgard"), is an unfathomably large and monstrous sea serpent or worm who dwells in the world sea, encircling the Earth (Midgard) and biting his own tail, an example of an ouroboros. As a result of him surrounding Midgard, the beast is referred to as the World Serpent. Jörmungandr releasing his tail is one of the signs of the beginning of Ragnarök.

Jörmungandr is said to be the middle child of the god Loki and the jötunn Angrboða. According to the Prose Edda, Odin took Loki's three children by Angrboða – the wolf Fenrir, underworld ruler Hel, and the serpent Jörmungandr – and removed them from Asgard (the world of the Æsir). The serpent Jörmungandr was tossed into the great ocean that encircles Midgard. There the serpent grew so large that he was able to surround the Earth and grasp his own tail. The old Norse thunder god, Thor, has a lengthy feud with Jörmungandr and the serpent is regarded as his archenemy. During Ragnarök, Thor and Jörmungandr engage in a ferocious battle, culminating in both of their deaths.

Atter is an older Germanic term for "poisonous bodily fluid", especially venom of a venomous animal, such as a snake, dragon or other reptile, but also other vile corrupt or morbid substance from the body, such as pus from a sore or wound, as well as bitter substance, such as bile. Figuratively, it can also mean moral corruption or corruptness; noxious or corrupt influence, poison to the soul, evil, anger, envy, hatred; as well as destruction and death.

Today, atter is commonly associated with the Norse mythology, where it plays an important role in various contexts – see subsequent section: § In Norse mythology. In Eddic poetry, both the sea serpent Jörmungandr, and the dwarf Fáfnir in dragon-hamr, are described as having attery breath. A similar creature from later Orcadian folklore is the attery stoor worm which was killed by the hero Assipattle, falling into the sea and forming Iceland, Orkney, Shetland and the Faroe Islands. As in the English tale of the Linton worm, the stoor worm is killed by burning its insides with peat.