The fallen angels, Metatron, and the Mark of Cain
19 episodes of Supernatural carry this thread, 19h in all — 16 of them are in The Essentials.
Arc membership is derived from each episode's own summary text, gated to the seasons the sources place each arc in. It shows where a thread is foregrounded, not every episode it touches.
In airing order
Dean is frantic after Sam collapses and the doctor tells Dean his brother won't make it. Desperate, Dean sends out an all points bulletin via prayer to every angel in the area, asking them to help his brother. In exchange, Dean will owe that angel a favor. Numerous angry angels race to find and kill Dean, but one, Ezekiel, gets to him first. Meanwhile, Sam fights for his life and must face some familiar demons to do so. Castiel struggles to survive without powers.
Ezekiel tells Dean a group of angels is hunting Castiel. Dean and Sam race to find their friend before the angels do. Meanwhile, a homeless Castiel is trying to make his way through the city and comes across some unexpected surprises.
Sam and Dean discover that the table in the Men of Letters house, has an ancient computer attached to it. They figure if they can get the computer on, they can track the angels. Who better to call for help than the best IT girl there is, Charlie ? While moving the massive computer, they dislodge a hidden hexbag that unleashes a magical green-webbed wall. Sam and Dean break through it and discover the one and only Dorothy from Oz. She had trapped herself and the wicked witch in the web years ago to stop the witch from finding the key that would open the gate that connected their world with Oz. With the wall broken, a massive and magical fight ensues that threatens to destroy the Men of Letters house.
Looking to reclaim his place as King of Hell, Crowley asks Dean for help finding "The First Blade," the only blade that can kill Abbadon. Knowing he owes Crowley a favor for helping Sam, and also wanting Abaddon dead himself, Dean reluctantly agrees and the two set off on a road trip. However, Dean and Crowley are completely unprepared when they find out who has the blade - Cain, the first son. Meanwhile, Sam is back at the bunker with Castiel who notices there is some leftover grace inside Sam. If they can extract it, they can use the grace to track Gadreel. The only problem is the procedure may do permanent damage to Sam.
Dean struggles with the after effects of the Mark of Cain. Meanwhile, Sam hears about a case where straight-laced people are turning into violent murderers. Sam suspects possession and suggests to Dean that they investigate, but Dean tells him to go without him. While interviewing the local townsfolk, Sam meets an elderly woman named Julia, who tells him the Men of Letters came to town in 1958. Josie tells Sam the story of a young man named Henry Winchester and his female companion, Josie Sands. While Sam is away, Crowley tests Dean.
Metatron attempts to get Castiel to join forces with him. Still furious with Metatron, Castiel refuses, which sets a surprising plan in motion. Meanwhile, Sam and Dean capture Gadreel.
Castiel captures one of Metatron's angels and asks Sam and Dean for help with the interrogation. Dean eagerly accepts, which doesn't go unnoticed by Sam. Meanwhile, Abaddon demands Crowley help her kill Sam and Dean. When he refuses, she reveals her shocking bargaining chip. Also, Castiel sets a meeting with Gadreel.
After a massive attack on the angels, Castiel calls Sam and Dean for help. As they leave, Dean's eagerness to bring the First Blade doesn't go unnoticed by Sam who is worried about the cost to his brother whenever he uses the Blade. Meanwhile, Castiel is shocked when he learns the angel that caused the attack was one of his followers and did it in his name. Dean discovers there is a conspiracy amongst Castiel's angel followers and at the heart of it is Tessa, the Reaper.
Dean feels the effects of the First Blade, and Metatron makes his move against humanity. Meanwhile, Sam, Dean and Castiel face shocking consequences when taking the fight to Metatron.
Crowley notices Dean is becoming more and more aggressive. Dean tells him that's the side effect of being a demon, but Crowley knows it's something more - the Mark of Cain needs to be fed. Meanwhile, Sam is captured by Cole, an angry man who blames Dean for his father's death years ago and wants revenge. Cole tortures Sam, hoping he'll tell him where Dean is hiding. Hannah sees how weak Castiel is becoming as his grace continues to fade, so she makes a bold choice and asks Metatron for help.
Sam continues his efforts to save Dean from the Mark of Cain. Crowley realizes Dean's demonic antics are starting to cause problems for him in Hell so he searches out an unlikely ally - Castiel.
After the massacre at the ranch, Dean, Sam and Castiel double their efforts to find a cure for the Mark of Cain but there is nothing in the lore that can help them. Desperate, Castiel comes up with an idea that Sam deems the "single worst idea I've ever heard." Meanwhile, Rowena continues to plot against Crowley, and Castiel tries to mend his relationship with Claire who is plotting revenge against Dean for killing her friend.
Looking to get Dean out of the bunker, Sam finds a case for him and Dean to investigate - people are disappearing into thin air with only their clothes left behind. Sam and Dean suspect fairies or angels, but the truth turns out to be much more shocking - Hansel, from Hansel and Gretel lore, is kidnapping people and turning them into their younger selves to placate the evil witch. Unfortunately, Dean finds this information out the hard way after he becomes Hansel's next victim and reverts to his 14 year old self.
Dean, Sam and Castiel deal with Cain's return. Crowley and Rowena continue to grow closer but when Crowley bails on plans with his mother to help the Winchesters, Rowena lets him have it.
Despite Dean's directive to forget about looking for a cure for the Mark of Cain, Sam continues to do research. Dean suspects Sam is hiding something but decides to focus on a new murder case where they run into Cole. The three team up and learn a Khan worm is infecting men and turning them into murderers. Unfortunately, the worm makes it's way into Cole.
Sam and Castiel follow up on a lead about the Mark of Cain. Dean and Rowena meet.
Charlie calls Sam and Dean to tell them she found the Book of the Dead which could help remove the Mark of Cain. The Winchesters race to her after she tells them she's been shot by a man named Jacob Styne who says the book belongs to his family and he won't rest until he gets it back. Castiel and Metatron take a road trip to find Castiel's grace but they hit a couple road bumps along the way.
Sam learns that former Men of Letters, Magnus, once built a magical box and Sam sets out to find it in the hopes it can cure Dean of the Mark of Cain. However, what he's not prepared for is the deadly alarm system Magnus set up on the box. When Sam opens the box, he puts both his and Dean's life in danger.
Dean makes a shocking decision regarding the Mark of Cain that would change not only his life, but Sam's too. Meanwhile, Crowley and Rowena face off and Castiel gets caught in the middle.