Leviathan

The Leviathans (also called Leviathan, The Old Ones, Chompers, or Levis for short) are a race of ancient, primordial monsters that served as God's very first beasts. Thus, they predate angels, humans, and the soul itself. The creatures proved to be too hungry and destructive, so God created Purgatory and locked them inside.

Characteristics
According to Death, the Leviathans were the first beasts created by God. Death describes them as "clever" and "poisonous". He also says he personally found them "entertaining". The Leviathan were known for their power and ravenous hunger and their purpose of satiating it by finding and taking sustainable food sources. Because of this, God was worried they might "chomp the entire petri dish." and locked them in Purgatory.

Leviathans are just as old as Eve and are related to her in some way. This is supported by Tsubaki's claims that the vampires, who are Eve's children, originate from Leviathans - to which Soren responds, "barely". Leviathans despise all other species and enjoy flaunting their superiority over their enemies. In general, Leviathans regard humans as little more than cattle and food. They also consider seven billion to be a limited resource demonstrating the masses they intend to consume. They do, however, have some respect for certain individuals, who possess a certain "spark" that the Leviathans are unable to copy. Their leader seems to admire these individuals for their ingenuity and progress, as well as their history of warfare and violence. Leviathans also consider angels to be lower than humans and garbage and would kill them all simply on principle if they weren't preoccupied with other things to do. They also hate the monster races sired by Eve, viewing them as competitors who are using up their food supply, to the extent where they genetically engineer their human complacency drug so that it poisons every monster as a side effect. Leviathans are also cannibalistic, eating other Leviathans for their failures (or just being annoyances). Their leader is known for his cruel punishment, known as "bibbing," in which he forces a Leviathan that failed him to eat itself. In general, Leviathans believe every other species to be inferior to them and consider themselves invincible, believing it impossible for humans, demons or angels to kill them. Because they are able to kill practically all other species, they consider themselves the top of the food chain and fear nothing but their superiors.

Though chaotic, it is established that Leviathans have some sort of hierarchy, with several of them taking orders from their leader.

On Earth, they take serpent-like shape underwater. Once within a vessel, Leviathans are able to shape-shift without taking a new one (provided they can acquire DNA). They seem to be able to change their hosts' biological structure, as they replace the vessels' blood with their black goo. While in a host, they can dislocate the jaw and reveal a mouth full of large teeth and a forked tongue. While they can devour almost anything, they seem to prefer humans to all others. They can also have particular tastes. Leviathans are also capable of eating human food, such as hamburgers and salads, but they find it disgusting. At least some of them enjoy the taste of other Leviathans.

Their true form in Purgatory, as perceived by humans, looks like a ball of black goo, much like the substance of their injured vessels on Earth. In this form, they have incredible mobility, as they have shown the capability to fly and land anywhere with a noticeable force, before taking humanoid form. Although this form still possesses the powerful jaws, the leviathans may choose when to reveal them. It should be noted that this may only be how they can manifest in front of human-like entities, like angels manifesting in vessels in front of humans, as they were originally created long before humans. Their original form has never been described before they were locked in Purgatory.

Powers and Abilities
As some of God's first creations, Leviathans are very powerful beasts, more powerful than most other creatures, and able to kill and/or eat just about everything else. It is notable that despite the Leviathans' status as much older than angels and their ability to kill the celestials with relative ease, Leviathans have yet to show any form of magical or psychic abilities besides flight when in Purgatory. Even the angels are powerless against Leviathans. This makes them rather unique as they are able to defeat creatures who possess far more abilities than they do. Before the Leviathan threat could be ended, it took the hunters, angels, demons and even other monsters to forge loose alliances with each other and combine their efforts.
 * Enhanced Senses - Leviathans have far superior senses compared to humans. Despite this, they are unable to see certain entities such as ghosts, but they can sense angels and demons.
 * Flight - In their true form in Purgatory, Leviathans have shown the ability to fly and land anywhere with extreme speed and force. In this state, they appear as meteors composed of a thick, black ooze.
 * Invulnerability - The only way for a hunter to kill a Leviathan is with a Bone Of Righteous Mortal Washed In the Three Bloods of Fallen. While use of witchcraft, Borax, decapitation or extreme physical force can disable and weaken them temporarily, they will recover from the damage in a matter of seconds to hours, depending on the extent of damage. More powerful Leviathans are less susceptible to Borax damage and can regenerate their wounds even quicker than others of their kind. Besides these methods, they are virtually indestructible.
 * Immortality - Being creatures much older than souls and angels, the Leviathans are immortal, in a variety of senses, in that by having an ageless lifespan, meaning they simply do not age, and can not be killed by but one mean, devised by God himself. Death reveals that the Leviathans were one of God's first creations, being before angels and humans. Leviathan are immune to literally every angelic and demonic weakness, and as stated above, can only be killed by that one way devised by God.
 * Possession - Leviathan can take over a human host's body and mind by a touch, the process alters the host biologically and appears permanent. Many of them can possess a single host at once. Not only can they possess an already occupied vessel, but they can eradicate the creature that is possessing the host they are invading.