There are things much older than souls in Purgatory. Long before God created Angels and man he made the first beasts: the Leviathans. I personally found them entertaining but he was concerned they'd chomp the entire Petri dish. So he locked them away. Why do you think he created Purgatory? To keep those clever, poisonous things out.
|Powers and Abilities||Super strength, not hurt by silver shot, regeneration|
|Vulnerabilities||Certain cleaning supplies (Borax), Beheading, Vessel can be stunned by witches' spells|
|Appearance||Black goo, or human with giant teeth and bifurcated tongue|
|Episode(s)|| 7.01 Meet The New Boss|
7.02 Hello, Cruel World
7.03 The Girl Next Door
7.05 Shut Up, Dr. Phil
7.06 Slash Fiction
Leviathans, also called the Old Ones, pre-date the creation of humans and angels, as well as the soul itself. They would also pre-date any beings with souls, such as monsters. They are therefore among the first creations of God.
Death describes them as immensely powerful and hungry. Before God went on to create more creatures, however, He felt that the Leviathans threatened to consume and destroy everything 'in God's petri dish'. For this reason, Death states that God created Purgatory and sealed the Leviathans inside to contain them and their hunger.
When Castiel opened the door to Purgatory again more recently and took in the power of the souls stored there, he also took in the Leviathans. With the help of Sam, Dean and Bobby, Castiel finally expels the souls back to Purgatory, but some of the Leviathans "held on". Castiel's Vessel is taken over by the Leviathans, and claim that Castiel is dead and no longer in control. They manage to guide Castiel's vessel into a nearby water catchment before the vessel disappears in a whirlpool, releasing them into the water supply.
The Leviathan are fervently hunting down the Winchesters, with unknown motivations as to why they are the prime targets. It has been discovered that certain cleaning supplies can harm them, and decapitation can stave off regeneration. Their current leader has been revealed to be Dick Roman.
The Leviathans have sharp teeth within an expanding jaw, and possess a bifurcated tongue. They feed on human internal organs - sometimes with cheese. All Leviathans absorbed everything that Castiel knew, as he was their original vessel.
They can take mimic a particular human form following contact, or if they obtain some DNA from a person. They are then aware of all that person's thoughts and feelings, and even hallucinations.Cite error: Invalid
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Powers and Abilities
- Possession: Leviathans can possess any human they come into contact with in their fluid form. This person becomes their physical embodiment of sorts. These shells bleed black ectoplasmic blood when injured and can reshape their jaws to display long sharp teeth and a two-pronged tongue.
- Shapeshifting: Once a vessel is taken they can reshape it into any human form they touch. They require only a fragment of DNA to tranform into a person; even lost hair will do. The creature possessing a little girl takes the form of a doctor by touching his arm. They can also absorb the memories of anyone whose form they take, much like a Shapeshifter, but without requiring the person to remain alive.
- Super Strength: They are shown to possess greatly enhanced strength; at their first meeting, they are able to hurl Dean and Bobby across a room with little effort.
- Healing/Endurance: Leviathans are extremely resilient, surviving multiple shotgun blasts with silver shot, including one to the head at point blank range. One also survived and reassembled itself after being crushed by a falling car. They can even re-attach their heads if decapitated. 
- Borax: Leviathans are highly susceptible to materials that contain the chemical compound Borax (Sodium Borate) such as household cleaning products. Contact with their skin results in an instant burning effect that can reduce them to charcoal-like husks if they are exposed to too much. It is suggested that they will eventually heal from the effects, however.
- Beheading: Removing a Leviathan's head disables them temporarily. But unless the head is kept far away from the body it will re-attach itself. The combination of burning them with Borax and then beheading them and separating the head and the body appears to be the only way to put them down indefinitely.
- Witchcraft: Leviathans show susceptibility to witchcraft. It can incapacitate them temporarily.
Leviathans on The Loose
The first leviathan appearance is when they take over Castiel. As Leviathans take the form of humans, they are referred to here by their human names.
- Leviathan Boss - Dick Roman
- Annie, who later becomes Dr Gaines.
- Two Members of the Stockville Sharks Swim Team, who later become a Head Nurse and a Hospital Administrator at Sioux Falls General.
- Valente - the FBI Leviathan
- Leviathan!Sam and Leviathan!Dean
- Eleanor Visyak was a creature released from Purgatory in 1937, however there is no evidence she was a Leviathan.
Whilst ingesting all the souls from Purgatory to became the new God, Castiel also took in the Leviathans. The Leviathans struggled for control, briefly taking control of Castiel's Vessel to smite people that even the now-wrathful Castiel didn't intend to, and causing his vessel to physically deteriorate and potentially explode.
With the assistance of Dean, Sam and Bobby, Castiel returns the souls to Purgatory. But "some" of the Leviathans hold on, and assume control of Castiel's vessel. Stating that Castiel is dead, the Leviathans attack Dean and Bobby with manic delight, remarking: "Oh this is going to be so much fun."
After having revealed themselves to Dean and Bobby, the Leviathans quickly realise they cannot remain inside Castiel's vessel as it begins to rapidly burn up and are forced to leave for the time being; they promise to return and finish what they started. Castiel/Leviathans staggers out into the open and submerge themselves in a nearby municipal water source, abandoning the vessel, leaving only Castiel's bloody trenchcoat behind.
The Leviathans then disperse to take separate vessels of their own by various means of exposure to the tainted water. One takes a girl whilst she drank from a public water fountain, for example. Two of the creatures infect a pair of gym students and use them to kill and devour two other students, leading Dean to their whereabouts. The creature possessing the girl meets with another Leviathan - Edgar - to discuss their next move and the "boss" they take their orders from.
The girl comes up with a plan to solve their feeding problem and steals the form of a doctor - Dr Gaines - at Sioux Falls Hospital. It then uses the position to acquire easy meals. Its act of cannibalism is witnessed by Sheriff Jodie Mills, who escapes the hospital with Bobby's help. Edgar arrives with the two gym student Leviathans, which take on the forms of members of staff. Bobby investigates one of the bodies and is confronted by the Leviathan Dr Gaines, who recognises him from before, when it was inside Castiel.
Bobby subsequently escapes the confrontation and races back home, informing the boys of his discoveries along the way. When they return to his house they discover it burned down and Bobby missing, however. Sam and Dean are then attacked by Edgar the Leviathan who informs them that they are considered "competent" enough that the head Leviathan wants them dead. The boys succeed in defeating it by dropping a car on it. Both suffer severe injuries, however, forcing Dean to call for an ambulance.
Back at Bobby's, the black blood of the Leviathan spills back into the body and its hand twitches as a sign of life.
Bobby arrives at the hospital where Dr Gaines works to rescue Sam and Dean. He tells Dean to meet him in the ambulance bay and tosses him a pair of crutches, then grabs the gurney holding an unconscious Sam. He stows Sam in the back of the ambulance and climbs in the driver's seat. Dr Gaines receives a call telling him the Winchesters are in the hospital, and with the Leviathan nurse he goes in search of the brothers. They spot the ambulance and toward it. Dean board the ambulance just in time and all escape.
Three weeks later, Bobby's contacts tell him that the leviathans at the hospital - Dr Gaines, the head nurse and an administrator - have disappeared. He says that other hunters are reporting similar attacks and that "they're like shapeshifters only a lot more into eating folk. And nothing can kill 'em."
A Leviathan has taken the form of an employee working for a credit card company called Chet, and has set up flags on credit cards with the Winchester brothers' known aliases. One is triggered when Sam uses the pseudonym Lemmy Kilmister in Whitefish Montana. Chet calls Edgar for guidance, and Edgar tells him to get after the brothers. He tracks down the location where the card was used, and finds Sam on the security footage. He calls Edgar again to tell him that he is on their trail, then pours a vat of hot cheese on the bound and gagged store clerk, and eats him.
Chet gets instructions by phone as to the Winchesters' location and says he's about a day and a half drive from Prosperity, Indiana. When he finds Sam and Dean, Dean shoots him, but his body simply pushes the bullet out. As Chet is about to attack, Don Stark enters and paralyzes him with some sort of spell, which he says will disable him for a day or two. he advises the Winchesters to put the creature in a bottomless hole.
Sam and Dean bind Chet with chains, and put him in the back of the Impala.
Chet is chained to a chair in the basement of Bobby's cabin, still weakened by Don Stark's spell. Bobby tries every method in the book to find a means to harm the Leviathan, but everything fails, much to Chet's amusement. Whilst preparing electric shock, Bobby brushes against Chet's arm, allowing him to take his form in order to mock him on a personal level. Bobby loses his cool and cuts Chet's head off, which seems to be the solution he was looking for.
Bobby later finds Chet re-attaching his head, however, and wonders how else he might harm the monster when something starts to drip through the ceiling and onto Chet, who screams in agony as the liquid burns through his skin. Bobby rushes upstairs to find his guest, Sheriff Jodie Mills, cleaning the floor with Borax products. He quickly douses Chet with the stuff and beheads him once again, this time keeping his head in a cardboard box and instructing Jodie to take it far away and "drop it in the drink".
Meanwhile, two more Leviathans have taken Sam and Dean's form and are going on a rampage from town to town, killing people en masse. This causes the authorities to place Sam and Dean on the top most wanted list and a manhunt begins, with two FBI agents taking a particular interest. The dopplegangers succeed in luring them into a trap and Sam and Dean are arrested by the police.
The Leviathans then infiltrate the police station and try to kill the Winchesters, but are caught by surprise when Dean uses Borax against them and decapitates them as informed by Bobby. With their heads missing, one of the FBI agents, actually a Leviathan, informs his "superior" of the situation and is advised to adopt new tactics in hunting the brothers.
Elsewhere, the Leviathan superior enters his limo and is greeted by the demon Crowley, who proposes an alliance between their two factions. The Leviathan coldly refuses his offer, however, considering Crowley a "hideous mutation" and not worthy of his attention. Crowley leaves, insisting the Leviathan keep his offering of infant-flesh muffins.
Leviathans in Lore
- In the Judeo-Christian tradition a Leviathan is a giant sea monster, described in the Book of Job Source
- The Talmud Baba Batra 75b states that the archangels Michael and Gabriel will be the ones who slay the Leviathan. Other legends say God will slay the beast. Source: Leviathan in Judaism
- Death also refers to the Leviathans as "The Old Ones" a term used by H.P. Lovecraft in the Cthulhu mythos to describe ancient powerful alien beings who come to Earth. A race called the Old Ones also features in Stephen King's Dark Tower Series.
- In Binsfeld's Classification of Demons, Leviathan is the name of Envy, one of the Seven Deadly Sins
- Sera Gamble and Bob Singer talk about Leviathans with AOLTV