It's over, the darkness has finally been defeated. Lucifer actually came through and kept his promise, but now there's an even bigger problem. Lucifer destroyed the darkness, but now he's free. And even worse, he's in Cas's body. He can bring back the apocalypse, or maybe even do worse. No one can even imagine all the things that he can do.
"I kept my promise," says Lucifer to Sam and Dean.
"Good, now give us Cas back" says Dean in a stern voice.
"Oh, you know, I would, but I kind of need a vessel to take over the world and since I'm already using Cas, I might as well just keep him. Plus, I don't trust you to not kill me" Lucifer says with a grin on his face. Tears are streaming down Sam and Dean's faces. They know they can't defeat Lucifer without killing Cas too.
"Just give him back, we won't try to stop you from taking over the world" lied Dean. He knows lying is the only way to save his friend.
"I wish I could believe you, but every monster, demon, and angel knows they can't believe Dean Winchester. Maybe you shouldn't lie so often" Lucifer says smiling. Both the boys know what they have to do. They've been planing this ever since they found out that Lucifer was possessing Cas. They just didn't know which of them would be strong enough to do it. They got an archangel blade in case this happened, but if they use it on Lucifer, it will also kill Cas.
"I'll do it" says Sam reluctantly
"No way" Dean says "last time you got a chance like this was when you needed to shoot dad to kill Azazel and you couldn't do it."
"This is different I-" Sam begins
"-It's not different" Dean interrupts. "I think I should do this."
"Ok." Same doesn't agree but he thinks it's best to not argue right now.
"Are you two ladies done chatting?" Asks Lucifer.
"Yeah" says Dean "And we're ready to kill your ass."
"Kill my ass? I'm flattered, but why not kill the rest of me too?" Jokes Lucifer.
"Go to Hell" says Dean .
"Been there, didn't really like it" says Lucifer .
"Well, don't worry" says Dean "you don't have to go back there, because I'm gonna kill you."
"Oh, are you" laughs Lucifer "you don't have the guts."
"I have the guts. I'm sorry, Cas, goodbye." Dean can barely get the words out through his tears. He plunges the blade into Cas's chest. Lucifer stands there wide-eyed. He can't believe Dean actually did it. He can't utter any words before he falls to the ground dead. Sam and Dean look at Cas's lifeless body.
"That's it" says Dean to Sam "I'm done."
"What do you mean you're done?" Questions Sam .
"I'm mean I'm done hunting, for good this time" yells Dean. "All I ever bring is destruction and death. It's all my fault, I'm the one that released the darkness. If I wasn't a hunter, the darkness wouldn't have been released and a lot of people would still be alive."
"You can't blame yourself for this" Says Sam to Dean.
"I can and I will" says Dean rudely. Dean gets in the Impala and drives off before Sam can say anything else. Sam stands there alone. The credits roll.