The Singer and Carver era covers Season 8 to Season 11, when Jeremy Carver and Robert Singer became co-showrunners following the departure of Sera Gamble. This era brings the show back to Heaven versus Hell roots in terms of story arc. Also, a key characteristic is that its seasonal arcs are more self-contained as opposed to the previous two eras. Whereas previous eras would transition to the next season as an intended goal or clear continuation of the preceding arc (i.e., all events leading to the Apocalypse, the opening of Purgatory and the use of its souls), this era often leads to the next season with unclear or surprise by-products of the previous conflict.
A common theme throughout the era, however, is the resurfacing of old and powerful supernatural organizations, particularly the Men of Letters. Several of the seasonal arcs are tied with the efforts of the Men of Letters before they became temporarily defunct. Other ancient organizations that appear throughout the era include the Thules, the Judah Initiative, the Knights of Hell, the Styne Family and the Grand Coven.
This era returns the series to the "Heaven versus Hell" conflict roots as a loose basis of its continuity between its seasons.
The ferocious beasts, called the Leviathans, are effectively apprehended, by the team effort of the Winchesters, Castiel, and Meg, by the death of their leader, Dick Roman. Dick is killed by Dean, with the Bone Of Righteous Mortal Washed In the Three Bloods of Fallen, the only implement in existence that can slay the Leviathan. The "kick" of the weapon, as Crowley calls it, sends Dean and Castiel to Purgatory - the realm where monsters prey on each other for eternity. There, Dean and Castiel team up with a vampire named Benny, and the three seek a portal, which can be accessed by humans, as a way out.
The first two were successfully completed by Sam and the third was almost to be done, when Sam was stopped by Dean. It seemed that the Scribe, Metatron failed to mention that Sam would die if he completed the third trial.
Meanwhile, Castiel became an unwilling pawn in the hands of Metatron. Castiel thought that completing the Heaven trials would lock the angels in Heaven and prevent their power struggle from reaching Earth. Instead Metatron banished every angel (except himself) to Earth, to live as humans.
Immediately after the fall of the angels caused by the angel Metatron, the Scribe of God, Sam falls into a coma from the effects of the trials to close the gates of Hell and almost dies. With Castiel unable to help (having lost his grace), Dean seeks the aid of another angel named Ezekiel, whose only option to save Sam from his wounds is to possess him. Dean accepts, and helps Ezekiel trick Sam into giving his consent against his knowledge. The three of them, with Kevin (and Crowley as their prisoner), work to find a means to put the angels back in Heaven and are also forced to deal with Knight of Hell, Abaddon, who is attempting to seize control of Hell in Crowley's absence and enforce her reign of Hell on earth. Also a newly human Castiel must continue to run from the thousands of angels who blame him for their setbacks.
Eventually, Metatron returns to Earth and Ezekiel is revealed to actually be the angel Gadreel, the protector of the Garden of Eden but failed by letting Lucifer into the Garden and was then punished by being locked away in Heaven's darkest dungeon. Metatron makes a deal with Gadreel to rebuild Heaven together by allowing certain angels to return which Gadreel, seeking to redeem himself accepts. Dean discovers Gadreel's deception and tries to warn Sam, but Gadreel takes full control and kills Kevin. Meanwhile, Castiel reclaims his powers by stealing the grace of another angel. Dean and Castiel team up with Crowley to expel Gadreel from Sam's body, but are forced to let the King of Hell go in exchange for his help.
Sam is furious at Dean's deception, and their relationship becomes strained as they struggle to deal with Kevin's death. Dean then teams up with Crowley in his hunt for Abaddon to find the only weapons that can kill a Knight of Hell, the First Blade and the Mark of Cain. Both of which Dean acquires from Cain himself, the original Knight of Hell as well as the trainer and creator of the rest of the Knights. Dean eventually finds Abaddon and kills her, but the Mark of Cain begins to affect him, bestowing him with great strength, but also a painful desire to kill. Meanwhile, Metatron continues to play God with the power of the angel tablet and starts enacting public miracles to become a deity to mankind and they begin to slaughter and praise in Metatron's name. Gadreel grows tired of Metatron's lies and defects, teaming up with Castiel to try and stop him.
In the season finale, Sam and Castiel worry about the Mark's influence on Dean, but eventually realize that he's their best shot at defeating Metatron. Castiel and Gadreel try to infiltrate Heaven to destroy the angel tablet and cut off Metatron's power, allowing Dean to kill the angel, but they are caught and put in Heaven's dungeon. Gadreel is forced to sacrifice himself as part of a spell to free Castiel. Meanwhile, Sam arrives too late to stop Metatron from stabbing Dean through the heart and his brother dies in his arms. Castiel manages to trick Metatron and take back Heaven, but chooses to spare his life and locks him in Heaven's dungeon though is dying from his draining grace. Sam tries to summon Crowley to fix Dean, but instead Crowley shows up in the bunker of his own choice where Dean's body has been placed and puts the First Blade back into his hands, which revives Dean as a demon.
Season 10 begins with Sam's frantic search for his missing brother, who is gone without a trace. The road to recovering the wayward Dean takes Sam down dark paths, with consequences that will shake the boys to their core. Along the way, Sam encounters a former US marine named Cole Trenton, who apparently knows Dean and wants revenge for Dean killing his father. Meanwhile, Castiel, with the aid of Hannah, has to pick up the pieces in the aftermath of Metatron's campaign. But with his grace failing and rogue angels still on the loose, Cass faces the ticking clock of his own mortality. After Sam learns that Dean is a demon, because of Crowley and the Mark of Cain, Sam eventually finds him, with help from Crowley, but so does Cole. While Dean and Cole meet and duke it out, Dean overpowers Cole and nearly kills him. But Sam manages to trap Dean with holy water and handcuffs carved with devils-trap symbols.
After giving Crowley the First Blade, Sam takes Dean back to the Men of Letters bunker to cure him. Castiel and Hannah are on the way to help but are delayed because of a rogue angel who injures Cass and tortures Hannah until Crowley shows and takes the grace of the angel and gives it to Cass. As Sam continues to cure Dean, he eventually escapes the devil's trap and tries to kill Sam but Castiel arrives, overpowers Dean and they continue curing him until he is finally human. As the season progresses, all-new threats emerge to once again push all of our heroes to their limits.
One such threat is the appearance of a very powerful witch named Rowena, who also turns out to be Crowley's mother and has the power to kill demons. He captures her, but she convinces him to let her live in his lair so they can reunite and be a family again. Meanwhile, while Castiel and Hannah still take care of the rogue angels, Sam and Dean work normal, everyday cases: dealing with not just demons but also ghosts, monsters (vampires, werewolves, and shapeshifters), deities, and again with angels. They also help out their friend sheriff Jody Mills. However, Dean is suffering from the effects of the Mark and Sam and Castiel desperately try to help him and find a cure, or at the very least, a way to get rid of it, even going as far as to torture Metatron.
Eventually, the Winchesters' friend Charlie returns from Oz as well and after combining her with her alter-ego, which she created from the Wizard of Oz to win the war in Oz, she agrees to help the boys find a cure for the Mark, but Dean is starting to give up and live with it. Hannah eventually abandons her mission with Cass and returns to Heaven. Later on, another one of those threats is the original bearer of the Mark, Cain himself, who has fallen under its influence as well and has started killing his descendants to rid the world of his tainted bloodline, which was stained the moment he killed his brother, Abel. After trapping him in a Devil's trap, Dean and Cain square off in a barn and Dean defeats him, stabbing him with the First Blade, while Sam, Castiel, and Crowley wait for him.
Dean has given up on finding a cure, but Sam still continues to search, going behind Dean's back and convincing Castiel to come see a psychic to contact the boy's surrogate father, Bobby, in Heaven, in order to get Metatron on Earth. After Sam and Castiel remove Metatron's grace, he reveals that he doesn't know how to remove the Mark, but he mentions that Castiel's grace still remains and takes him to it. However, Metatron finds and takes the Demon tablet but not before Cass manages to get his grace back and returns to his status as a Seraph. Sometime after, Charlie contacts the brothers about a possible way to remove the Mark with a book called the Book of the Damned, a very powerful dark magic book that was in possession of the Styne Family, an ancient family that was the inspiration for Mary Shelley's Frankenstein and has consisted entirely of magic practitioners mixed with vastly superior surgical skills for multiple generations.
Sam calls upon Rowena, Castiel, Charlie, and Crowley to read the book and find a way to remove the Mark. However, one of the Styne members kills Charlie and Dean becomes hell-bent on revenge, giving in to the Mark. After the Styne family captures Dean, Dean falls under the Mark's influence, kills the entire Styne family, savagely beats and almost kills Castiel when he tried to stop Dean, and disappears. He heads to a Mexican restaurant and summons Death, the Pale Horseman himself, to kill him. However Death reveals that the Mark not only makes Dean unkillable, thus even he can't kill Dean, he also tells Dean that if he even removes the Mark without passing it along to another host, it will unleash the Darkness, a powerful and destructive primordial force that predates the Earth, the Universe, the Archangels, and even God. He also tells Dean that after God and the Archangels fought the Darkness in a horrifying war, God created the Mark of Cain to lock away the Darkness, and sealed it away using the Mark, which was originally borne by Lucifer himself. The Mark revealed itself as a curse after corrupting Lucifer as he became jealous of humans, who was banished to Hell for his crimes against humans and God. Lucifer passed the Mark on to Cain before his imprisonment in his Cage, who then transferred it to Dean.
Death tells Dean that he can send Dean somewhere else so that the Darkness remains sealed and the Mark remains intact on a host, but he has to kill Sam. After Sam and Dean fight, Dean is ready to kill Sam with Death's Scythe, but ironically, Dean kills Death at the last second due to his love for his brother. After that, the Mark is removed from Dean's arm, thanks to the spell from the Book of the Damned cast by Rowena, who later pins Castiel and Crowley and puts Castiel under her attack dog spell so he will kill Crowley. Elsewhere, the Darkness is released.
Following the release of the Darkness, Sam sees that Dean is gone and goes to look for him, finding him in a field where Dean had a conversation with the Darkness, which apparently turns out to be, or at least, takes the form of a woman in a black dress. Dean says that she thanked him for setting her free. As Sam and Dean drive away from the old diner, they find a Sheriff's deputy named Jenna Nickerson and head to a hospital to help others. Upon arriving there, they find people with black veins which Dean calls Rabids, people who have been forced to let their inner darkness out. This being an effect of the release of the Darkness. They also find a human baby in the mix named Amara.
As Dean, Jenna, and the baby make it out of the hospital, Sam using himself as a diversion gets infected and starts to sprout black veins. Eventually, after he prays to God for help, he encounters a Reaper named Billie who is apparently disappointed that Death is dead and says that the next time Sam and Dean are killed, she will ensure that they stay dead and even that she will toss them into the Empty, a place no soul nor entity nor being can escape from.
Back at the hospital, Sam finds a cure for the "Rabids": Holy Fire. And he makes a ring of holy fire and cures a few others as well. The baby turns out to be Amara as a child as Dean finds the Mark of Cain on her. Amara sucks out Jenna's soul and she becomes soulless, like how Sam once was. After Crowley kills Jenna, Dean attacks him as he plans to take the baby, but fails as the baby walks out and quickly grows up.
While Sam and Dean work on ordinary cases involving ghosts, monsters, and even along the way Imaginary Friends, the brothers and Cass also start searching for ways to defeat the Darkness. Meanwhile, Sam's visions start to become more revealing, as he starts to have visions of Lucifer's Cage. Castiel eventually finds Metatron, who has started a business of catching what he calls reality but is also miserable about his current status. After beating and throwing him, Castiel askes Metatron what he knows about the Darkness, considering he was God's scribe. Metatron reveals that in order to created the Earth, Heaven, even the entire Universe, in order to start creation itself for that matter, God didn't only imprison the Darkness, but he had to betray and sacrifice her and Metatron says that the Darkness is the only thing God's really ever known, his only kin...his older sister.
Although Dean hates the idea, they intend to go the Limbo, the deepest place in Hell, and speak to Lucifer about the Darkness. They enlist the help of Rowena to summon him. And when Dean goes to find Amara at a church where she attacks people, Sam awaits outside of a supernatural cage in Limbo surrounded by powerful fire and covered in enochian sigils, and eventually, Lucifer himself appears in the cage, manifested as his original vessel, Nick. He and Sam talk about the Darkness and how to stop her while Amara and Dean talk about her issue with her brother. After three angels find her and try to attack her, she kills them all and summon a powerful smiting attack that knocks her out. In Hell, Lucifer says that he will help them stop her, but only if he possesses Sam and returns to Earth. Sam says no and Lucifer extinguishes the fire, erases the warding, and summons Sam in the cage and still tries to make him say yes, especially when he reveals that it wasn't God who sent Sam the visions, but it was Lucifer. When the Darkness was set free, it damaged the cage and allowed Lucifer to reach out to Sam. Though Sam strongly considers, he still says no and Lucifer starts to attack him.
Rowena reveals that Lucifer came to her in a dream and said he needed her help to get Sam to him and she would join him. But, after Cass finds Amara, having survived the attack, she summons him to Dean with a messages and eventually, as Rowena conjures the spell to banish Lucifer, Dean and Cass head down to the cage and help Sam survive in a fight against Lucifer. As Lucifer also kills Cass, Lucifer is banished by Rowena. After leaving Hell, Cass apparently stays and reveals the before Lucifer could be banished back to the Cage, Cass said yes to him and Lucifer possessed him. After freeing and then killing Rowena, Lucifer takes back Hell and makes Crowley his slave, calling him "Puppy".
Meanwhile, Sam and Dean eventually find possible ways to defeat Amara, using Hands of God, powerful objects said to contain the power of God himself. Even after finding one, Sam, Dean, Rowena, and Crowley try to get Lucifer out of Castiel's body but it fails and he absorbs the power of the Hand of God and uses it on Amara, but it fails and he is taken against this will by Amara. And she starts to torture him to draw out God. And as fate would have it, eventually, God himself finally returns. After having a conversation with Metatron and reconciling with Lucifer, he decides to help stop her. And though his original plan of weakening her with Witches, Demons, and Angels and trying to imprison her again using the Mark fails, Amara fatally attacks God.
Although they think Amara seems impossible to destroy, God tells Sam, Dean, Cass, Crowley and Rowena that she does have one weakness: Light with the power of 10,000 suns. So they collect souls to build a bomb. And Dean is the one to bear it. After talking with Amara, whom starts to feel bad about attacking and nearly killing her brother, Dean convinces her to forgive him. So she summons him to a garden area and the two reconcile and decide to go off together for a while. However, Amara leaves Dean with a thank you gift and he finds his mother alive and resurrected. However the the bunker awaits a British woman who banishes Cass with a banishing sigil and shoots Sam, leaving his fate unknown.