The first beings created by God, the Leviathans were imprisoned in Purgatory when God became worried they would consume His other creations. In 2011, they were freed when Castiel opened Purgatory and absorbed all the souls there to become the new God. The Leviathans eventually took over Castiel, but when Castiel proved unable to contain them, they left the angel's body and escaped into the world, possessing many people.
- Achilles' Heel: The only way to kill a Leviathan is to stab it with the bone of a righteous mortal dipped in the blood of a fallen angel, the King of Hell and a father of the fallen beasts (an Alpha monster, the first of a monster species). This weakness is fairly hard to use against them because of meeting the above conditions and that they managed to keep it a secret until "Reading is Fundamental." As for their other weakness, see Weaksauce Weakness below. The only other known way to kill one is have it eaten by another Leviathan, or being forced to eat itself.
- Until "Reading is Fundamental," even they didn't know that weakness. They only knew that their one weakness was written down somewhere.
- Witchcraft and decapitation slow them down at least. Prior to learning their true weakness, Sam and Dean tended to defeat them with a combination of borax and decapitation.
- Always Chaotic Evil: They are an entire race of hateful, smug, treacherous (Joyce ate four of her assistants simply for annoying her, to say nothing of Dick) and ravenous killing machines who, unlike demons and monsters, never displayed anything in the way of qualities or traits that could be construed as being even remotely positive (while one did help the Winchesters, he only did it to spite his hated boss, the aforementioned Joyce). While they do suffer from a perpetual hunger that is described as being beyond all human comprehension, this was never played for sympathy, and was entirely unrelated to things like their sadism and genocidal narcissism (they planned on wiping out all demons and monsters, seeing the former as "garbage" and the latter as contemptuous "mutts"). And they have always been like this, as Death reminiscently refers to them as "poisonous things" and the reason why God locked them all away in the first place was because of His justified fear that they would have zero compunctions with devouring literally everything else in existence.
- Ax-Crazy: Its clearly obvious from their actions and demeanour while possessing Jimmys body that they are extremely unhinged.
- Big Eater: As in, eating humans and everything else. As in, 7 billion humans is not enough food for them.
- Cool and Unusual Punishment: Standard operating procedure for Leviathans who fail their tasks is to be forced to eat themselves! Now remember that, due to their Healing Factor, the sheer act of doing so becomes a Fate Worse than Death since, if they slack off at all whilst devouring their own flesh, it will simply grow back and they'll be forced to do it all over again!
- Demonic Possession: They take control of Castiel, or at least hijack his vessel Jimmy, at the end of the Season 7 premiere. Noticeable was the change in tone of voice from Castiel's deep monotone to one that sounds more like an insane version of Misha Collins's normal voice.
- In the following episode, Jimmy's body is about to be destroyed by their power, so they disperse, mass-possessing an unknown number of people across the country.
- Eldritch Abomination: The first things God created before everything else. Their hunger forced God to seal them away. But now they're back out thanks to Castiel..
- Fantastic Racism: They consider demons to be lower than garbage and would kill them all if given the opportunity though they are willing to cooperate with them if it serves their interests. They don't seem to have a high opinion of angels, either. Humans they actually admire somewhat, for their ingenuity and progress, as well as their history of warfare and violence, but they still view them primarily as food. They generally regard every species as inferior to them, partly because they are capable of killing all of them (yes, even angels) and thus consider themselves the top of the food chain. They also despise the monster races sired by Eve (vampires, werewolves, etc), viewing them as mutts who're taking up their food supply. Because of this, they've specifically engineered their human complacency drug so that it poisons monsters as a side effect.
- Faux Affably Evil: Most of them tend to come across as this.
- Hero Killer: In "Survival of the Fittest," it is revealed that the Levianthans have killed an entire garrison of angels (that is, earthbound angels; they don't seem capable of invading Heaven or anything).
- Humanoid Abomination: When a leviathan possesses a host, the host body essentially becomes their physical form itself unlike other entities capable of possession who instead simply hijack their host bodies instead of outright assimilating them.
- Human Resources: Dick Roman wants to turn all of humanity into complacent and stupid food sources for the Leviathans. They consider seven billion humans to be a limited resource.
- It's Personal: They got Castiel and Bobby killed. Dean's method of coping with this is to single-mindedly devote himself to taking them down in a way that'd make John Winchester proud. (Or seriously concerned.)
- Jerkass: This is their least bad quality.
- The Mind Is a Plaything of the Body: a subtle version, but each of the Leviathans shown, seems to adopt at least some mannerisms of the body they have copied, Dick Roman being super friendly and a Large Ham, which seems to be the Original Roman's personality as nobody seems to see it as a big change, with other creatures that possess/copy humans generally falling into Out-of-Character Alert, also, the Fake Sam & Dean, have conversation about switching who is each, because apparently, their copying also gave them The Boys' feelings and mindsets that they have to keep pushing down, irritating the two greatly.
- More Teeth than the Osmond Family: As one can tell from the picture above.
- Names to Run Away from Really Fast: Just watch Dean's reaction after Death first mentions them in "Meet the New Boss." He has never even heard of them before, but he knows they are bad news.
- Nigh-Invulnerable: They are virtually unkillable, being able to recover from any injury from any weapon, save for their one Achilles' Heel.
- No Name Given: Like Demons, most are only known by the names of their hosts. Given their age, they might not even have names. Certainly not ones humans could pronounce anyway.
- Ominous Obsidian Ooze: Their true form incorporates black sludge.
- Sealed Evil in a Can: Let's put it this way—God created Purgatory "just to lock those poisonous, clever things away."
- Slasher Smile: Their default expression, and pretty much the only smile they have.
- Smug Snake: Many of them, Roman especially.
- Space Whale Aesop: In "Slash Fiction," Bobby didn't see the point of Sheriff Jody Mills cleaning up Rufus' cabin. After testing every monster weakness imaginable on a captive Leviathan, the thing gets burned by borax that leaked into the basement while Jody was scrubbing the floor above. A lot of lives could have been saved if the average hunter was more into household chores.
- Stupid Evil: Their inflated opinion of themselves didn't help their cause when they refused to work with demons or any other supernatural beings, deeming them inferior. This was very crucial in turning the Alpha Vampire and Crowley against them which proved really crucial to their leader's demise.
- To Serve Man: Not much of a surprise on this show, but their first instinct/threat regarding anything seems to be "Eat him."
- Token Good Teammate: Somewhat. Among the Leviathans is George, who is bullied by Joyce Bickleby, a Bad Boss superior-ranking lady Leviathan and, due to frustration, ends up helping the Winchesters by giving them crucial information. This still doesn't stop him from wanting to feed like the other evil Leviathans; he is just as selfish as the rest, but that happens to mean he is willing to sell them out (to a degree, since he doesn't think his info will make any difference) if it serves his interests.
- Villains Blend in Better: They seem to be adapting to the modern world fairly well, considering they predate everything on Earth. Justified since they gain the memories of anyone they possess or shapeshift into (though they need DNA for shapeshifting).
- Voluntary Shapeshifting: Combined with their Demonic Possession, Leviathans can be virtually anybody.
- Weaksauce Weakness: They're immune to every monster-slaying technique and implement that's ever appeared on the show, but are damaged by borax, a common ingredient in cleaning supplies that burns them like acid. They were just as surprised at the revelation of this weakness as everyone else.
- Wham Line: "This...is going to be...so much fun."
- Also, "Cas is gone. He's dead. We run the show now."
- And from "Reading is Fundamental", "Rock beats scissors...Leviathan beats angel."
Potrayed By: James Patrick Stuart
- Bad Boss: Played with; when two Leviathans draw attention to themselves early on, Dick accepts that they've just come out of Purgatory and has The Dragon beat them up and then put them to work where they'll be useful. But as the Leviathans become established in human society, he makes it clear that any more security breeches won't be tolerated and kills those who stuff up in nasty ways.
- Big Bad: For Season 7. As the leader of the Leviathans, Roman plans on placing chemicals in corn syrup so that humanity will be rendered helpless for the Leviathans to feed off. He's also responsible for Bobby Singer's death, which makes the Winchesters even more driven to stop him.
- Broke Your Arm Punching Out Cthulhu: Turns out him, the head Leviathan, drags the nearby people who killed him into Purgatory.
- Corrupt Corporate Executive: Dick Roman, the man whose form was assumed by the head Leviathan, was one of the richest men in America. Since the death of the real Dick Roman and rise of the Leviathan who literally stole his identity, the latter slowly started becoming the most powerful businessman via multiple massive corporate takeovers.
- Empty Eyes: Even when he is smiling, his eyes are absolutely devoid of expression.
- Exact Words: While promoting his additive, he proclaims he's concerned with having his food help Americans "live longer and tasting better", when asked if he meant "the food tasting better", he nods and says that's exactly what he meant.
- Fight Off the Kryptonite: Dick Roman pulls off a villainous version of this trope by enduring a sustained barrage of borax-based cleaning products by the Winchesters long enough for him to catch up to them and fire the shot that ultimately killed Bobby Singer.
- Hero Killer: In "Death's Door", he successfully killed Bobby Singer.
- Killed Off for Real:After being stabbed with what is described in Achilles' Heel. However, it is an example of Broke Your Arm Punching Out Cthulhu, as killing the head Leviathan brings those who killed it into Purgatory.
- Once an Episode: The show threw out a Dick pun at least once every episode. "The rise of Dick", "Grabs himself some Dick", "Dick is hard to beat"...
- Slave to PR: As Dean notes to him, being in a celebrity's body means he can't just kill Dean himself, particularly in public.
- Smug Super: So righteously confident in his abilities, that Dean procuring the one weapon that could kill him had him more amused than anything.
- Villain with Good Publicity: Pretty much everyone outside of the main characters and some of the side characters believe that Dick Roman is a businessman and a nice one too. In one episode, someone endorsed him for the Presidency. This is justified though with the implication that Dick makes in "There Will Be Blood" that he is replacing media figures with Leviathans that are in human forms, so he can look popular in the public eye. Of course, most people do not know about his plan.
- You Have Failed Me: When Dr. Gaines' plan to use drugged fast food to make humans more complacent draws unwanted attention, Roman kills him—by forcing him to eat himself. Turns out this is standard protocol—unless you've really managed to piss Dick off, in which case he'll just eat you himself.
Potrayed By: Benito Martinez
- The Dragon: Edgar seems to fulfill this role for Dick Roman and generally acts as the second-in-command of the race.
- Fight Off the Kryptonite: Edgar shows some resistance to borax-based cleaning products in later appearances as well.
- Hero Killer:
- Delivers a worse beating on the Winchesters than they've ever received on the show, with the (possible) exception of Lucifer. Afterwards, Dean actually calls an ambulance, which—as the Television Without Pity recapper pointed out—he's never done before in the entire series' run.
- In "Reading is Fundamental," he is revealed to be able to slaughter angels with disturbing ease.
- Nigh-Invulnerable: Survived having a car dropped on him.
Potrayed By: Olivia Steele-Falconer (Annie) and Cameron Bancroft (Gaines)
- Evil Genius: Dr. Gaines serves this role for the Leviathans. Dr. Gaines has synthesized a food additive that is very addictive, and consumption leads to weight gain, and dampens people's emotional responses until they are very apathetic. It is latter refined to avoid the adverse effect discussed under the You Have Failed Me entry below.
- OOC Is Serious Business: The differences between the mannerisms of the real Dr. Gaines and the Leviathan who kills and replaces him is enough to alert Jody Mills that something is wrong and has to escape from the hospital.
- Villainous Legacy: He dies early on in the season, but his food additive, with some refinements, is retained for use as part of the Leviathan's endgame.
- Wrong Genre Savvy: After watching some episodes of Dr. Sexy M.D., he repeatedly quotes lines and does mannerisms of people from that show, ignorant of the fact that is not how real surgeons act like. Though in his latter appearance in "How to Win Friends and Influence Monsters" he stops trying to copy the Dr. Sexy M.D. characters upon learning more about the workings of the Human world.
- You Have Failed Me: He is "Bibbed" when it is discovered that 0.03% of cases consumption of food with the additive resulted in "hyper-adrenalised cannibalism," thus breaking their golden rule that "there are no monsters".
Potrayed By: Sean Owen Roberts, Jim Beaver
- Scarily Competent Tracker: Even with the Winchesters constantly on the move, he manages to intercept them in Prosperity, Indiana.
- Sinister Surveillance: One of many who serve the Leviathan cause by monitoring everything that could lead them to the Winchesters, including credit card transactions.
- Smug Snake: Even when captured, he gleefully gloats about the superiority of the Leviathans. Even while Bobby was torturing him to find any weaknesses the Leviathans have, Chet continually taunts the hunter. Its not until he is exposed to borax that he finally loses his cool.
- What an Idiot!: In-universe example in that he smugly mentions how the Winchesters left behind genetic material and a money trail that allowed the Leviathans to find everything they needed on them.
Potrayed By: Olivia Cheng
- Dark Action Girl: Downplayed. As Dick Roman's personal secretary, she isn't involved with any combat or other out of office related duties, but she will fight when necessary to prevent the Winchesters and their allies from achieving their goals (those being Bobby Singer reporting back to the brothers about the Leviathans plans in "How to Win Friends and Influence Monsters" and preventing Kevin Trans' escape in "Survival of the Fittest").
- Dark Chick: Serves as Co-Dragons with Edgar in that whereas he is a Front line manager checking on operations in person, Susan is Roman's assistant who relays his plans with lower ranking Leviathans to keep them on track.
- Sexy Secretary: Serves as one of these for Roman.
Not-Sam Winchester, Not-Dean Winchester
Potrayed By: Jared Padalecki and Jensen Ackles
- Continuity Cavalcade: Invoked by them to provide a pattern for their massacres with the intention of drawing The Winchesters out of hiding so that either they can kill the brothers or get them arrested and killed in custody afterwards.
- Criminal Doppelgänger: Two Leviathans take on the shape of Sam and Dean and go on a high-profile murder-crime spree with the intention of implicating the real Winchesters.
- Evil Twin: Each of the them to the real ones.
- Homage: Their scene in the St. Louis diner is straight from Pulp Fiction, minus the swearing and endearments. Lampshaded by Bobby.
- "The Reason You Suck" Speech: Dean's impostor delivers one of these to Sam in which he accuses the Winchesters of stupidly fighting for a lost cause.
Potrayed By: Mary Page Keller
- Bad Boss: Her irritability led her to eating four of her previous assistants who displeased her and ordering around her latest one like a dog on a leash. It's really no surprise when the current assistant switches sides by helping the brothers dispose his superior when the opportunity presented itself and giving them information on the Leviathan's intents.
- To Serve Man: The ultimate goal of Bicklebee's Portland, Oregon based part of the Leviathan's plan. She was going to gain ownership of the properties in the area so as to establish a Hospital which would be used to cure cancer and by extension make sure their livestock (humans) would be fit for them to eat.