The Singer and Carver era.
When Jeremy Carver came on board as showrunner in season 8, he planned story arcs till the end of season 10.
"We really are building some foundation for a storyline that can carry us on for at least a few more years. There’s a note card in my office that says “The end of season 10” right there.
I have that note card; that note card exists in fantasy right now. I don’t know if there’s going to be a season 10 but, you know, that’s what keeps me getting up in the morning."
- Jeremy Carver
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.