Leviathans are scaly reptiles that live in the sea and resemble scaly snakes with fins and tails. The medium of the species are five or six times the size of an average man, the larger ones twice that. They are cousins of the whales except for the fact that leviathans have smaller skulls, more serpent like appearance and wider tail flukes.
Leviathans are generally very intelligent and friendly to man once trained, unlike dragons - though they are predators which could easily bite a man in two, were they not friendly. And unlike dragon riders, those of Leviathan often become affectionate towards their animals and form deep bonds with them. Cornelu named his leviathan Eforiel, after the first woman he had a relationship with, a fact only he knew.
It is for their intelligence, many Derlavaian navies make use of the creatures to sink enemy ships, for example the Royal Sibian Navy. Sibians are especially well known for their skill in using Leviathans, and have the reputation of "making the most" of them.
Specially trained sailors, with a magic enabling them to survive underwater, ride on a leviathan and swimming under enemy ships, stick an egg under the target's keel. They have an advantage over the ships, which are restricted to moving on ley lines while Leviathans and their riders range freely over the seas.
Leviathan riders are also used as couriers, able to pass secret messages over long distances and to smuggle spies or refugees in or out of an enemy country.
The name "Leviathan" is taken from the Bible where it is mentioned in several Old Testament books (most famously in the Book of Job) as a legendary sea monster. Bible scholars have long debated whether the texts intended to describe a shark, a whale, a dinosaur, or some other zoologically definable creature. In modern Israeli Hebrew, "leviathan" is simply the word for "whale".
Leviathans take the place of submarines in the Darkness series' World War II metaphor. However, they are of lesser strategic importance than submarines were in WWII. Algarve does not try to launch an all-out Leviathan campaign against Lagoas as Germany used U-boats against Britain. Leviathans, for all their uses, are essentially auxiliaries which do not decide wars.
- Cetus, the archetypical "sea serpent" of Ancient Greek mythology.
- Tiamat, the Babylonian analog.
- Sifan, a sea serpent from Shetlandic folklore.