Amongst gods earliest creations, the leviathans are maybe the single most dangerous species God has ever created. They eat and eat and eat without being able to stopped. At least that is what it seemed like back when the winchesters encountered them for the first time.
God created the Leviathans around the same time he created the archangels. They were the first beasts ever created, for what purpose they were created wasn’t really clarified and remains a mystery. They were also dangerous enough to make Death himself dubbing them “entertaining”. Seeing how hungry they seemed, God feared for them to “chomp the entire petri dish”, thus locking them away in purgatory, though it is not clear if purgatory already existed or if God created purgatory exclusively for them.
As a Plan B God created besides the angel and demon tablet a third one, which contains a description for a weapon to permanently get rid of a Leviathan.
Once freed by Castiel, they scattered around North America, possessing people and starting to take plans into action. Their leader Dick Roman, the only one they’ve ever had since the beginning of their existence, got into a company named SucroCorp, where he and other Leviathans started working on high fructose corn syrup, which had a DNA-altering additive in it to turn mankind into fat and mindless drones, ready to be eaten once fat enough. In order to conquer North America and to keep their food source for them alone, they’ve also started hunting other species with a taste for humans. They made the food poisonous for vampires and other creatures, which caused a huge disruption between monsters and forced them to group up with hunters to get rid of the Leviathan plague.
With the help Kevin Tran, who awakened as a prophet the second the Leviathan tablet was stolen from Dick Roman and freed from its prison and more help from other creatures, demons and angels, Dean Winchester and Castiel finally killed Dick Roman, the leader of the Leviathans, which caused a big explosion that trapped Dean and Castiel in purgatory. According to Crowley, the rest of the Leviathans scattered around the planet, having no leader for the first time and no one to rely on. Their real fate is unknown, even though some of the Leviathans returned to purgatory, hunting Dean, Castiel and later Benny in Purgatory.
The Leviathans are more powerful than angels and demons and capable of eating almost every other species and monster on the planet. They are presumably immortal and can possess other humans with ease. By coming in contact with just one strand of DNA of a person, Leviathans are able to shapeshift into the person they got the DNA from and are thus able to shapeshift into every being in existence. They are also able to conceal their true form to other beings, as they could hide from angels to catch them by surprise and kill them. Raphael also couldn’t see or sense the leviathans inside castiel, when he absorbed every single soul in purgatory, further clarifying the power Leviathans have.
Their only weaknesses are beheading, slowing them down for some time, and for some reason Borax, burning their skin and weakening them.
★ The Leviathan Blossom, which blooms around the corpse of a Leviathan, is required for the spell God used to imprison the Darkness Amara.
★The Leviathans have developed enough of a taste palette to have different food preferences. Dick Roman having a famous chef prepare food for him or Chet liking human meat doused in cheese.