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Crowley approaches Dean for help with finding the First Blade, the only known weapon that can kill Abaddon. Following clues, Dean and Crowley discover thatDean's fatherhad once worked with a hunter named Tara who spent years searching for the Blade. Finding Tara, they cast a locator spell she found that leads them to the "retired" demonCain(Timothy Omundson), who had trained—and then killed—all the other Knights of Hell except Abaddon, who had escaped his wrath. Cain refuses to help them, even letting in a group of other demons to fight Dean and Crowley, who kill them. Cain reveals that he wants revenge on Abaddon, as she murdered his wife, but is unable to exact it himself because he is bound by his promise to his wife not to kill anymore. Cain also reveals that the spell led them to him becausehis Mark(the brand Lucifer marked him with when he turned him into a demon) is the source of the Blade's power, and he can transfer it to someone worthy. Despite Cain's warnings of an unknown but terrible cost, Dean agrees to bear the Mark so that he can kill Abaddon. Cain asks for Dean to kill him after killing Abaddon, then sends Dean and Crowley to safety as he single-handedly defeats a swarm of demons that arrived to kill them. Having picked up on Crowley giving himself away in small but tell-tale ways, Dean confronts Crowley on having been deceiving and manipulating him all along. This includes Crowley having allowed Tara to be questioned, tortured, and killed by demons so that they would find him and Dean, who could prove himself worthy to Cain for the Mark. However, Dean can't retaliate against Crowley yet because he still needs Crowley to retrieve the First Blade from the bottom of the ocean. Running parallel to this plot is Sam and Castiel's attempts to harvest the part of Gadreel's grace that is still inside Sam, so that they can use it to track the angel. Removing the grace causes Sam to start reverting into the state he was before Gadreel possessed him, but Sam refuses to let him stop until they get all the grace they need, even if it means his death. Due to his own brief humanity, Castiel empathizes with Sam's desire to make up for the people who've died from the choices he, Sam, Dean and his wife bella and thire daughter grace have made. However, he refuses to risk killing Sam, and instead finishes healing Sam and eliminates the last of the grace. The spell turns out to be unsuccessful because they didn't get enough grace. before Castiel leaves to track Metatron, who he believes holds the key to fixing everything he has another night with bella