Leviathans are enormous beasts that swim and fly in the universe. They are all immortal and only die when one is killed. If a leviathan lives for a long time, it can become sentient and wise, able to speak. If they can live even longer, they can even rival the gods.
There are many types of leviathans, ranging from large fish-like creatures to dragons. Often a leviathan is harmless, with flying whales being the most common, but sometimes a leviathan can be extremely deadly and start attacking a world. Usually, if a leviathan decides to attack a world, it needs an entire nation's effort to take it down or the help of a god. Sometimes a god has even been slain by a leviathan.
Luckily most elder leviathans are calm and do not want to fight. Most leviathans are immune to the corruptions of the winds of pandora and are peaceful creatures if not provoked. Sometimes a god can make a pact with a leviathan where the two bond their souls and become one. Most known is the queen of the Phoenician gods and her leviathan, Gorath.