dean + bobby.
When I went off to Purgatory, Sam hooked up with a vet for a year, and I dragged him away from that to hunt again. He left Kevin on his own, and when we found the kid he was stir-crazy. We got him the demon tablet, and he found trials to close up Hell that Sam started even though I wanted to do it. I talked him out of finishing the third one because it would kill him, and the ritual we used for the third trial was a demon cure. He almost cured Crowley before he had to stop.
Metatron messed with Cas and tricked him out of his grace so he could board up Heaven, and all the angels who fell started splitting into factions and fighting their wars on Earth instead. Sam almost died from the trial anyway, so I tricked him into saying yes to an angel I thought was Ezekiel. Turns out it was Gadreel, the guy who was supposed to guard Eden and let Lucifer give him in the slip. Gadreel started working with Metatron and used Sam to kill Kevin. Crowley helped Sam kick Gadreel out of his head and everything went to shit between us, obviously.
Crowley went all soft because we nearly made him human, and this old Knight of Hell, Abaddon, started going for the crown. We had to pick the lesser of two evils, so we threw behind Crowley and let him out of our basement so he could go head to head with her, but nothing could kill Abaddon but the First Blade that Cain used to kill Abel. The Blade didn't work without the Mark of Cain seared onto someone, so I took it, found the Blade, and went after the bitch.
Metatron got his grubby mitts on the angel tablet and basically made himself God. Cas We didn't depower him in time, so when I went after him with the Blade, I lost. And I died. The Mark of Cain turned me into a demon. I mean, to be fair it was kinda doing that already, but dying triggered a real transformation. Eventually Sam and Cas rounded me up and cured me, and they've been looking for a fix ever since.