Millennia ago, when Cain discovered that his brother Abel was talking to Lucifer, he made a deal with Lucifer where his soul would go to Hell in exchange for Abel's going to Heaven. Lucifer demanded that Cain kill his brother and send him to Heaven as well and Cain did so, crafting the First Blade for the job. Lucifer cursed Cain with the Mark of Cain, a curse that kept the Darkness at bay and caused a bloodlust in the wielder. Eventually, the atrocities the Mark caused him to commit was too much for Cain and he killed himself with the First Blade. However, the Mark of Cain refused to let Cain die and resurrected him as a very powerful demon.
As a demon, Cain created the Knights of Hell under the orders of Lucifer, creating an order made up of purely evil demons who committed atrocities across the Earth for centuries. Cain eventually fell in love with Colette Mullen and retired to marry Colette who asked him to give up his evil ways in exchange for her love. Cain complied, but the Knights of Hell kidnapped Colette to try to force Cain to return to them. In response, Cain took up the First Blade, the only weapon that could kill a Knight of Hell and slaughtered the Knights in revenge and to rescue Colette. Eventually, only Abaddon was left and she possessed Colette to get at Cain. When Cain refused to return to his evil ways and join her, Abaddon started snapping Colette's bones. Cain attempted to kill Abaddon with the First Blade, but she fled Colette's body, leaving Cain to kill Colette instead. At Colette's request, Cain didn't chase down Abaddon for revenge, letting her live and throwing the First Blade to the bottom of the Mariana Trench, the deepest part of the ocean he could find as the First Blade couldn't be destroyed.
Abaddon eventually resurfaced in 1958, creating a "demon factory" where she stole souls to transform them into demons and slaughtering the Men of Letters. After being defeated while searching for the Key of the Order of Letters in 2013, having followed Henry Winchester there through time, Abaddon was put back together by Sam and Dean Winchester in hopes of curing her to complete the Trials of God. Abaddon escaped, but not before she learned that Crowley was now the King of Hell to her disgust. Abaddon reemerged after the Winchesters had captured Crowley, intending to kill him and take over Hell for herself. However, Sam doused Abaddon in Holy Fire, forcing her to flee her vessel Josie Sands and foiling her attack on Crowley.
After her attack on Crowley was foiled, Abaddon had a loyal demon restore Josie's body for her to use as a vessel. Abaddon then began gathering a following to take over Hell for herself and then eventually the world, attacking the Winchesters in Eugene, Oregon before fleeing when Gadreel revealed himself nearby. With Crowley captive in the Men of Letters bunker, Abaddon took advantage of his absence to start reshaping Hell to her desires, calling in soul deals early in order to boost the soul count in Hell. After helping to free Sam from Gadreel, Crowley was set free and informed Abaddon that he intended to wage a campaign against her for rule of Hell, fighting for the "hearts" of the minor demons to see who would rule Hell.
Shortly after separating from Sam after saving him from possession by Gadreel, Dean is approached in a bar by Crowley. Crowley tells Dean he needs Dean's help to kill Abaddon by locating the only weapon that can kill a Knight of Hell: the First Blade. Crowley explains that he's been hunting for the First Blade for decades and the closest he got to it was when one of his demons, Smitty, learned of a protégé demon of Abaddon's who claimed to have knowledge of the First Blade. However, John Winchester had captured the demon before Smitty could get to him and Crowley suggests that there might be some information on the First Blade in John Winchester's Journal. Dean reluctantly checks the journal which confirms that John did capture the demon Smitty had been after, but is unable to find much more. At Crowley's insistence, Dean admits that a series of numbers in the margin refer to John's Lock-Up and suggests that John may have left more information on the case there. Though reluctant to trust Crowley, Dean agrees to check the lock-up in hopes of finding a clue on the location of the First Blade. Unknown to Dean, one of Abaddon's demons spots them and begins following them.
At the lock-up, Dean searches through John's case files and finds one on his capture of Abaddon's protégé. Dean learns that John worked with another hunter named Tara and exorcised the demon after an interrogation, during which the demon mentioned the First Blade to John and Tara. However, the file is empty of anything else so Dean decides to have them locate Tara in hopes that she's still alive and can tell them something more about the location of the First Blade.
Dean and Crowley locate Tara at a pawn shop she runs and Dean introduces himself as John Winchester's son. However, Tara feels a pain in her knee that lets her know when a demon is near and aims a shotgun at Dean and Crowley who is left stuck in a devil's trap hidden under a rug in the pawn shop. Tara initially believes that Dean is possessed since he is working with Crowley, but he is able to prove he isn't when she splashes him in the face with holy water. Dean explains that Crowley is helping him on a job and that Tara and John had captured a demon who knew something about the First Blade and they need to find the weapon. Dean continues on to explain that they need the First Blade as they're hunting a Knight of Hell and that they're not all dead as was believed since Abaddon has returned. Convinced, Tara shows them her notes on the case, explaining that the demon they captured told them that the archangels used the First Blade to kill the Knights of Hell. Neither of the two had ever heard of anything like the Knights of Hell or the First Blade and John thought the demon was lying to save himself. After they exorcised the demon and went their separate ways, Tara started looking for the First Blade, believing that such a weapon could give a hunter an edge. However, after searching all over the world, all she could find was a location spell for the First Blade she couldn't finish. When Crowley learns that Tara couldn't finish the spell because she couldn't find the last ingredient which is essence of Kraken, he reveals that he can help as he has "a warehouse full of Kraken essence in Belize." At Dean's insistence, Tara reluctantly breaks the devil's trap so Crowley can get the Kraken essence. While Crowley's gone, Tara warns Dean about working with him, but Dean says that Abaddon is way worse and he can deal with Crowley after killing her. Crowley returns with the essence of Kraken and Dean performs the location spell which points to a spot in Missouri. Though they offer Tara the chance to join them, she refuses to work with Crowley and remains behind as Dean and Crowley head to the location the spell indicated.
Awhile after Dean and Crowley leave, the demon from the bar enters Tara's pawn shop looking for information on Dean and Crowley. Recognizing him as a demon, Tara shoots him in the face with her shotgun, but is unable to defeat the demon as her devil's trap is still broken from when she freed Crowley. The demon brutally tortures Tara for information, learning where Dean and Crowley went before killing her.
At the farmhouse the spell pointed to in Missouri, Crowley senses something dark and after seeing a nearby beekeeper, tells Dean they need to leave right away. Crowley explains that the beekeeper is actually Cain, the Father of Murder who Crowley tells Dean they need to be "a world" away from. However, Cain blocks him from leaving and Dean and Crowley wait inside his house to talk to him with Cain blocking Crowley's ability to teleport. Dean asks Crowley about Cain and Crowley explains that after Cain killed Abel, he became the deadliest demon to walk the face of the Earth and killed thousands. Crowley further explains that Cain was "the best at being the worst" before he disappeared and everyone believed or at least hoped he was dead. Cain enters the room and asks what the King of Hell and a Winchester are doing looking for him and literally shuts the nervous Crowley up when he won't stop talking to Dean's amusement. Dean explains that they're looking for the First Blade to kill Abaddon and asks for Cain's help due to the bad situation in the world. When Cain continues to demand to know how they found him, Dean tells him that the spell was for the First Blade and no one else knows they're there. Cain refuses to help them and tells them to leave, but Dean refuses to leave without the First Blade. Dean tells Cain that Abaddon is the last of the Knights of Hell and asks why Cain would care if Dean killed her if he was retired. To Dean's surprise, Cain reveals that he created the Knights of Hell himself, training the entire order, including Abaddon. As Crowley knew this, Dean is left annoyed at Crowley not sharing this with him. However, Cain shares with Dean something that no one but Abaddon knows: the archangels didn't kill the Knights of Hell, Cain himself did. Cain refuses to tell Dean why he killed the Knights of Hell and orders him to leave and never return. Outside, Crowley wants to leave but Dean recognizes that the First Blade is their only hope of killing Abaddon so he decides to wait until Cain leaves, go back into the house, find the First Blade and steal it.
Later, after Cain's gone, the two reenter his house to find the First Blade. Unnoticed by either of them, the demon that followed them from the bar locates the house and recognizing it as Cain's, calls for backup. While searching the house, Dean finds a picture of Colette Mullen and sees that she is wearing a similar ring to Cain's. Dean realizes that Cain got married and that's why he retired and informs Crowley who is unable to find any sign of the First Blade. At that moment, Cain returns and casts a spell that locks them all inside the house. As Crowley hands over the picture, the demon from the bar arrives and reveals that he's working for Abaddon. The demon calls out that all he wants is Dean and Crowley and the demons will leave Cain alone if he hands them over. Dean realizes that there's too many demons to fight and starts to barricade the entrances to the house with Cain telling him that the spell he cast on the doors will keep out the demons for a while. However, Cain refuses to help them fight Abaddon's demons, telling them that he's retired and they exposed his home to the demons. To Dean's annoyance, Cain leaves them to deal with the demons and tells them that if they survive the fight, they can join him for a meal before he leaves again for good.
Dean and Crowley barricade the entrances to Cain's house, but Cain continues to refuse to help them. Amused by Dean asking for help, Cain lets in three demons, including the one that killed Tara to fight Dean. Dean pulls out the demon-killing knife and engages the three demons single-handedly. Dean quickly manages to kill the demon who had been following him and Crowley, but the other two demons overpower him, taking the knife and pinning him to the table while Cain sarcastically comments on the fight. Dean eventually manages to kick the female demon off of him and restart the fight. Despite being outnumbered, Dean manages to get the female demon in a hold, retrieve the demon-killing knife from her and kill her with it. Dean then fights the remaining demon one-on-one, eventually gaining the upper hand and pinning him to Cain's kitchen table. Once the demon is pinned, Dean stabs him through the neck with his knife, killing him. At the same time Dean fights the demons, another breaks into the front room where he faces off with Crowley. Despite the demon knocking Crowley to the ground, Crowley easily kills him with an angel blade.
With the demons dead, Dean asks what Cain's purpose in letting the demons into the house was, if Cain was testing him. Cain simply tells Dean that he felt connected to Dean from the very beginning, seeing them as kindred spirits and very much alike. When Dean points out he saved his brother, not kill him as Cain did with Abel because you don't ever give up on family, Cain asks him where Sam is then. Refusing to answer Cain's question, Dean demands to know where the First Blade is. To Dean's shock, Cain tells him that he no longer has the First Blade and thus cannot give it to him. Crowley demands to know how that's possible since the spell brought them to Cain, but Cain explains that the spell didn't bring them to the First Blade, but rather the source of its power: Cain himself. Cain reveals to them a brand on his arm and a shocked Crowley explains to Dean that its the Mark of Cain. Cain confirms this, telling them that its from Lucifer himself and the Mark of Cain and the First Blade work together: without the Mark of Cain, the First Blade is useless. Dean realizes that the First Blade is the weapon Cain used to kill Abel and Cain explains that Abel wasn't talking to God as was believed, but rather Lucifer. Seeing that Lucifer was corrupting his brother, Cain offered Lucifer a deal: Abel's soul in Heaven in exchange for Cain's in Hell. Lucifer accepted on the condition that Cain send Abel to Heaven himself so Cain crafted the First Blade and murdered his brother to save his soul. Cain then became a Knight of Hell and created more on Lucifer's orders, leading the Knights of Hell for centuries in bringing chaos and darkness. Cain tells them that he continued committing unspeakable acts of evil until he met Colette who knew who and what he was and loved him unconditionally still. As Colette's only request was that he stop killing, Cain retired. However, when the Knights found out, they kidnapped Colette to get Cain to come back to them. Instead, Cain took up the First Blade once more and slaughtered all of the Knights of Hell except for Abaddon. When he reached Abaddon, he found her possessing Colette and when Cain tried to stop Abaddon from killing Colette, she fled Colette's body, leaving Cain to accidentally kill his wife instead. While Cain had wanted revenge he explains, Colette's last request was for him to stop the killing so he left Abaddon alone and ended his killing spree. Though Dean sympathizes with Cain's story, he refuses to give up on trying to get Cain to tell him where to find the First Blade so he can stop Abaddon forever. However, Cain continues to refuse, even stabbing himself with the demon-killing knife to no effect when Dean threatens him with it to prove a point. When Dean confirms he never gives up on anything, Cain tells him that he does and teleports away, leaving Dean and Crowley alone in his house with dozens more demons arriving to attack. Outnumbered, Crowley promises to stay with Dean to help him fight as long as he can before he will flee.
After teleporting away, Cain visits Colette's grave where he tells Colette he's tried to see himself as she did but he knows who and what he is and asks her to stop watching over him. Cain then teleports back to his house to the surprise of Dean and Crowley. Cain offers to transfer the Mark of Cain to Dean if he wants, telling Dean that the Mark can be transferred to someone who's worthy. Cain tells Dean that while he can kill Abaddon with it, wielding the Mark of Cain comes with a great burden, but Dean doesn't care as long as it allows him to kill Abaddon. Wishing Dean luck, Cain transfers the Mark of Cain to Dean who asks where Cain stashed the First Blade. Cain tells them that nothing can destroy the First Blade so he dropped it into the deepest ocean as it was the only way he could keep his promise to Colette. Cain tells Dean to find the First Blade and kill Abaddon, but when Cain calls him, to find Cain and kill him with the First Blade as well. When Dean asks why, Cain teleports Dean and Crowley outside without a further answer. As Dean and Crowley flee in the Impala, Cain lets all of the demons into his house, locks them in with him and engages them in battle, ultimately killing them all.
Arriving at the edge of some water, Dean asks Crowley how they will locate the First Blade. Crowley tells Dean that while Dean can't search the bottom of the ocean, he can and he will find it and bring the First Blade to Dean. As Crowley prepares to leave, Dean confronts him, having seen Crowley kill the demon he fought at Cain's house and then sit back and watch Dean fight the other demons on his own. Dean realizes that Crowley knew about the Mark of Cain and the history between Cain and Abaddon and played him. Crowley confirms this, telling Dean that Cain never would've given him the First Blade, but Crowley knew he would give it to Dean. Dean accuses Crowley of knowing they were being followed by a demon and letting it happen, which Crowley confirms, stating that he did it so that Cain could see Dean in action. Though his actions got Tara killed, Crowley shows no remorse for it, leading to Dean promising to kill him after he's done with Abaddon. Crowley is unconcerned and tells Dean that even with the First Blade, they will need all the help they can get against Abaddon. Though angry, Dean reluctantly accepts that he still needs Crowley's help and sends him off to find the First Blade.
Weeks after their encounter with Cain, Dean is unable to reach Crowley about his search for the First Blade and begins to get annoyed and worried. After getting a strange voicemail from Crowley, Sam and Dean decide to summon a Crossroad Demon to get more information. The two are able to summon a Crossroad Demon into a giant devil's trap they paint in the middle of a crossroads and are surprised when the demon turns out to be possessing reality star Snooki. Though the demon is reluctant, they eventually get her to talk and she tells them that the last she heard, Crowley was in the Western Pacific. Snooki tells them that things in Hell are getting crazy with even Crowley's loyalists starting to want to sign on with Abaddon who is going to make her move soon. After getting what they can from Snooki, Sam exorcises her to her annoyance as she believed they would release her instead.
Now hooked on human blood, Crowley relies on a demon named Lola to get him blood and other things he needs, unaware that she's really a spy for Abaddon. Lola reports to a demon named Aldo about her progress, feeling that Crowley can't function without her anymore and telling Aldo about Crowley and the Winchesters search for the First Blade which she learned of from a voicemail. When Lola returns to Crowley's hotel room he reveals he knows she's been spying on him as another demon told him and attacks her. After Lola derides Crowley's claims that he could help her, Crowley kills Lola with an angel blade.
After returning to the bunker, Sam looks up information on the Mariana Trench and feels that it makes sense that that would be where Cain was referring to when he said he dropped the First Blade into "the deepest ocean." Sam wonders if Crowley found the First Blade and is double-crossing them, but Dean doubts it as he needs Dean to power the First Blade and kill Abaddon. Sam tells Dean that Crowley is only useful until they have the First Blade and suggests killing him afterwards which Dean agrees to. Moments later, Crowley calls Dean for help after discovering Lola's deception and killing her.
Arriving at Crowley's hotel room, Sam and Dean handcuff him to a chair after discovering his human blood addiction. Sam and Dean remind Crowley of how they were counting on him to find the First Blade for them while his followers are counting on him to kill Abaddon and reclaim Hell. Crowley eventually tells them that Lola betrayed him to Abaddon and he doesn't know if he had told Lola about the First Blade or not. Knowing that if Crowley had told Lola than she had told Abaddon, Sam and Dean decide to force Crowley to quit human blood "cold turkey" and lock him in the bunker's dungeon while they try to piece together where the First Blade is.
In the dungeon, Crowley reports to Sam that he swept the Mariana Trench where he found that the First Blade was no longer there, having been picked up by an unmanned submarine. Afterwards, it was stolen by a research assistant who sold it to Portuguese smugglers who then lost the First Blade to Moroccan pirates in a poker game. Crowley tells Sam that the First Blade was then bought from the pirates by a man named Andre Devlin who has been trying to sell it.
Sam and Dean set up a meeting with Devlin, growing annoyed with Crowley's continuing antics in which he tries to steal candy from a vending machine. Finally, Devlin arrives and refuses to talk when Sam and Dean identify themselves as FBI, telling them he's just a conduit between the buyer and the seller. However, Crowley takes matters into his own hands by briefly possessing Devlin to get the information they need from him. Afterwards, the Winchesters let Devlin go and Crowley informs them that the First Blade is at the National Institute of Antiquities in Kansas City, Missouri. Having tracked the First Blade there as well, Abaddon has two of her demons possess security guards to search the vaults. The two kill a research assistant that interrupts them, but are unable to locate the First Blade as the vault is empty. Before leaving, the two make their vessels shoot and kill each other.
The Winchesters travel to the institute where they pose as FBI agents investigating the deaths of the guards and the research assistant. The head detective shows them the security footage and explains that the vault the demon-possessed guards were robbing was where the institute kept new, rare acquisitions while they're being examined. However, the vault had been empty for weeks so nothing ended up being stolen. After the detective leaves, the Winchesters realize that the vault was likely where the First Blade was put when it was first brought to the institute and the demons killed their vessels and reported back to Abaddon when they couldn't find anything inside. Sam and Dean then question the curator, Doctor McElroy who confirms that the First Blade was not on display and tells them that no authenticated item by that name was ever on the premises. Dean threatens to compel her to tell them what she knows since the First Blade was smuggled into the United States in violation of several international treaties and Doctor McElroy reluctantly tells them that she did acquire an item believed to be the First Blade, but couldn't reliably authenticate it as such. As a result, when she received a confidential offer several weeks before to buy it, she sold it and the guards had no idea. Doctor McElroy tells them that all she knows about the buyer is that he calls himself Magnus, but she doesn't know his real name or where he lives. After Doctor McElroy leaves, Sam reminds Dean that Albert Magnus is the name the Men of Letters used when they wanted to go incognito and suggests that Magnus is a surviving Man of Letters.
Hoping to find out if there are any surviving Men of Letters, Sam and Dean turn to Crowley and ask him about the Men of Letters Massacre of 1958 and ask if anyone other than their grandfather and Larry Ganem survived. In exchange for being let out of the dungeon and being allowed to have a drink, Crowley tells the Winchesters that a rogue member had been tossed out of the Men of Letters before the massacre and thus could still be alive. After learning that a Man of Letters could still be alive, Sam and Dean go through the list of the entire membership in 1958 and find that all of the names match those who were killed. When they point this out to Crowley, he reminds them that they are looking at the active membership at the time of the massacre and someone who was thrown out wouldn't have been considered an active member. The Winchesters then look through the files of the Men of Letters who were expelled from the organization, eventually locating one named Cuthbert Sinclair. Cuthbert had designed most of the warding that keeps the bunker safe and was so good at magic, he was made Master of Spells right after his initiation. However, his work eventually got crazy to the point that the Men of Letters leadership called it "eccentric" and "irresponsible." Cuthbert had proposed multiple projects over the last two years that he was apart of the Men of Letters before finally being kicked out in April, 1956. Sam and Dean realize that Cuthbert is Magnus as he would've missed the massacre since he was no longer part of the Men of Letters at the time. Crowley tells the Winchesters that he never knew Magnus' name, but he did hear that a member of the Men of Letters had been kicked out. Crowley had searched for Magnus in hopes he could help Crowley get into the bunker, but he could never find him. The Winchesters then ask Crowley to take them to where he had searched.
Crowley leads the Winchesters to a clearing where his demons had tracked Magnus to, but is unable to sense Magnus' presence if he is there, suggesting that Magnus is heavily warded. Sam remarks that Magnus is supposed to be a genius at warding and Dean believes that Magnus may be willing to talk to them since they are legacies. Guessing that Magnus may be listening as he's bent on hiding, Dean has Sam speak to try to convince Magnus to let them in. Sam introduces them as Henry Winchester's grandsons and Men of Letters and tells Magnus that they know what happened to him with the Men of Letters. Sam tells Magnus that they don't necessarily agree with what happened to him and suggests that he tell them his side of the story. In response, Magnus opens a door into his lair and they enter. As they make their way through the lair, two vampires attack them. Sam and Dean each kill one and Magnus congratulates them over an intercom before meeting with them in person.
Magnus apologizes for having the vampires attack them, explaining that he wanted to see what Sam and Dean are made of. Magnus tells them that his lair is right where they were standing in the clearing and is invisible. Magnus confirms that he's Cuthbert Sinclair though he hasn't gone by that name in fifty-seven years and that he's immortal as "there's a spell for everything." Magnus tells them that his lair contains the "greatest collection of supernatural antiquities and rarities on the planet" and asks about them being Men of Letters as he'd believed the organization destroyed in 1958. Sam explains that while they're legacies, they're primarily hunters, amusing Magnus given how much the Men of Letters looked down upon hunters and Sam and Dean now having access to the bunker. Magnus admits that while he's not fond of the Men of Letters, he was always fond of their grandfather who he mentored before being kicked out and who would visit him occasionally in secret, getting in the same way they did by calling out to him. Dean tells Magnus that they have come because Abaddon is looking to take over Hell and Magnus tells them that he was always trying to convince the Men of Letters to do more to rid the world of monsters, but they refused, feeling it wasn't their place. Dean explains that they're after the First Blade to kill Abaddon which they heard Magnus has and shows him the Mark of Cain when Magnus starts to point out the fact that the First Blade won't work without it. Sam tells Magnus that if Abaddon takes over Hell, she wants Hell on Earth which not even Magnus can hide from and Magnus reveals that the First Blade has been on display behind them the entire time. Dean asks for Magnus to loan them the First Blade so they can kill Abaddon, but instead, Magnus uses a spell to teleport Sam back outside.
Magnus explains to the enraged Dean that Sam is fine and he "separated the ordinary from the extraordinary" so that he could add Dean and the Mark of Cain to his collection as he did the First Blade. Magnus asks Dean to join him as his companion, stating that its gotten lonely over the years and offering Dean the chance to learn all of his secrets. However, Dean just finds the offer creepy and refuses, planning to leave. When Magnus tells him that there's no windows or doors, Dean pulls out his machete to make his own, causing Magnus to cast a spell to make it glow red hot, forcing Dean to drop the weapon. Dean tries to force Magnus to let him out at gunpoint, but by that point, Magnus has picked his pocket and gotten Dean's gun and welcomes Dean to his collection at gunpoint.
Magnus chains Dean to a post, causing Dean to tell Magnus how he derides the Men of Letters for sitting around and doing nothing but is worse than them himself. Ignoring this, Magnus forces the First Blade into Dean's hand, causing the Mark of Cain to activate and making Dean feel a rush of power before he forces himself to drop the First Blade. Magnus is pleased with Dean's reaction to the First Blade's power, telling him that it will get better over time as Dean gets used to the feelings from the First Blade and the Mark of Cain and will even welcome them. Magnus is excited at the idea that together, he and Dean will be unstoppable with Magnus' magic and Dean's ability to wield the First Blade. Dean refuses, asking what Magnus will do since he can't kill him without rendering the First Blade useless since he needs Dean and the Mark of Cain to power it. In response, Magnus casts another spell to begin brainwashing Dean to be under his control.
Outside, Sam informs Crowley that Magnus has Dean and begins going through the files on Magnus to find anything that can help, growing annoyed with Crowley's antics. Finally, Sam locates something in one of Magnus' proposed projects: a spell to make the entire bunker invisible, all physical points of entry eliminated "and entrance would only be gained by spell." Finding the spell to access such a place, Sam sends Crowley to get the needed ingredients. Once they have them, Sam casts the spell and opens a door into Magnus' lair which he and Crowley use to access it.
Making their way through the hallway, Sam spots Magnus and holds a silver knife to his throat, demanding he take Sam to Dean. Magnus leads Sam into the main room where the still-chained up Dean calls out a warning to Sam. The Magnus Sam encountered turns out to be a shapeshifter and assumes another form to attack Sam. Sam kills the shapeshifter with his silver knife, but the real Magnus gets the drop on him with Dean's gun and chains him up. Instead of forcing Dean to help him through a spell, Magnus begins torturing Sam to get his agreement. As Dean desperately struggles, he notices Crowley who had remained unnoticed by Magnus and Crowley is able to sneak in and release Dean.
Magnus notices Dean's release and panics, going to kill Sam before Dean can save him. However, Dean grabs the First Blade and decapitates Magnus, killing him. However, killing Magnus causes the Mark of Cain to drive Dean into an uncontrollable state of rage. Before he can harm anyone else, Sam manages to talk Dean down and get him to drop the First Blade, allowing Dean to regain control of himself.
Returning outside, they find the Impala having been searched by demons. Dean realizes that Abaddon's demons searched it for the First Blade, but couldn't find it or Magnus' lair due to his warding. To Dean's further anger, he discovers a message carved into the side of the car in Enochian from Abaddon to Crowley: "Be afraid. Your Queen." As Dean examines the damage, Sam reminds him that they had agreed Crowley was only useful until they got the First Blade and the two turn to deal with him now that they have it. As the two start to move, Crowley pins them to the car with telekinesis and thanks them for forcing sobriety from his human blood addiction on him. Crowley takes the First Blade, explaining that he realized that along with Abaddon, he must be someone they want dead as well. As a result, Crowley intends to hold onto the First Blade until they locate Abaddon and will only give it back when they can kill her. Crowley then teleports away, leaving the frustrated Winchesters behind.
After being able to wield the First Blade briefly, Dean becomes obsessed with getting the chance to do so again and relentlessly hunts for Abaddon. While Sam is initially against this, he joins Dean after discovering Abaddon's plan to create a demon army loyal only to her by stealing souls.
Eventually, Crowley's most trusted demons betray him to Abaddon who kidnaps his son Gavin from the past to force Crowley to cooperate. With Gavin as leverage, Abaddon is able to force Crowley to help her lure the Winchesters into a trap where she can kill them both and destroy the First Blade. However, after helping them get the First Blade, Crowley manages to warn Dean of the trap secretly. In the battle that follows, Dean manages to kill Abaddon with the First Blade, ending the quest he and Crowley started out on when they began their hunt for it.
Dean is eventually killed in battle with Metatron and due to the Mark of Cain's power, comes back as a demon and a Knight of Hell himself as Cain did when he killed himself in biblical times. Sam eventually reverses the transformation, but Dean still retains the Mark of Cain. The two begin a search for a way to remove the Mark of Cain and end its power over Dean, but inadvertently release the Darkness when they succeed.
After killing Abaddon's demons, Cain falls back under the power of the Mark of Cain and begins killing again. Believing his bloodline to be tainted, Cain begins an effort to wipe it out despite his bloodline being "legion." With the help of Crowley and Castiel, the Winchesters confront Cain and Dean kills him.
- Final time John Winchester's Journal is seen being used on a case to date.
- Against Abaddon's demons, Dean shows himself to be an immensely skilled fighter, enough to impress Cain himself. While Dean has defeated multiple demons before, he has never done so single-handedly before.
- During their encounter, Cain implies that Dean has a massive reputation for his fighting skills against demons. After seeing him kill three demons single-handedly, Cain tells Dean that he lives up to his reputation. Cain also mentions that Dean's reputation states that he's brave but impulsive.
- When Cain gives Dean the Mark of Cain, he says he's transferring it to Dean. However, its later stated that Cain still has the Mark and just gave Dean a copy.
- Before teleporting Dean and Crowley to safety, Cain tells Dean that he will call upon Dean to kill him for "what I'm about to do." Dean later discovers that Cain went back to his old ways and is forced to kill him. When Dean reminds Cain of what Cain told him, Cain just shrugs it off.
- During the Werther Project, Sam discovers that Cuthbert Sinclair was kicked out of the Men of Letters for creating the Werther Box to hide Nadia's Codex.
- ↑ First Born
- ↑ Brother's Keeper
- ↑ Do You Believe In Miracles?
- ↑ First Born
- ↑ Mother's Little Helper
- ↑ As Time Goes By
- ↑ Clip Show
- ↑ Sacrifice
- ↑ Devil May Care
- ↑ Heaven Can't Wait
- ↑ Road Trip
- ↑ Road Trip
- ↑ First Born
- ↑ Blade Runners
- ↑ Mother's Little Helper
- ↑ King of the Damned
- ↑ Do You Believe In Miracles?
- ↑ Soul Survivor
- ↑ Brother's Keeper
- ↑ The Executioner's Song