This story begins when Lucifer/Sam is about to confront Michael/Adam in the cemetery. Just moments after Lucifer kills Cas for throwing a Molotov Cocktail at Michael. Most of this chapter is canon. The dialog is taken from the show transcript of the episode 'SWAN SONG'. Where you see the asterisk(*) is where the story diverges from canon. The rest of the story will be my original work.
I do not own the characters from Supernatural. And I am not profiting from this work.
The thing that killed Cas and now walked slowly toward Dean, used to be a great hunter. Used to be a legend in the world of the supernatural. Used to be Sam Winchester. Sam was a kind and gentle soul. Unless, of course you were a monster. Now he was the 'Father of Lies', 'The Embodiment of Evil, 'The King of Hell'. Sam was trapped inside his own head, screaming, scratching, and pounding to break free while Lucifer slaughtered his family.
"Sammy, can you hear me?", Dean breathed. Lucifer kept walking, ignoring his question.
"You know... I tried to be nice... for Sammy's sake", Lucifer said. "But you... are such a pain... in my ass". Lucifer grabbed Dean and threw him against the windshield of the Impala and it shattered. Bobby shot Lucifer in the back and when Lucifer turned around, Bobby shot him in the chest. Lucifer sneered up at the old hunter and made a twisting motion with his hand. Bobby's head twisted violently, snapping his neck. He slumped to the ground, lifeless.
"NOOOO!", Dean screamed.
"Yes", Lucifer cooed. He grabbed Dean's legs and pulled him off of the Impala. Lucifer punched him hard in the face and he fell back into the side of the car and spat blood at Lucifer's feet.
"Sammy?", Dean croaked. "Are you in there?" Dean pleaded with Sam to show some sign that he was able to take control.
"Oh, he's in here, all right", Lucifer snarled. He punched Dean again with all the force he could muster. "And he's gonna feel the snap of your bones!" Lucifer landed another hard blow to Dean's left eye. "Every single one!" He jerked DEAN to his feet and growled in his face. "We're gonna take our time". Lucifer pummeled Dean mercilessly, rage seeping from every pore. Dean, his face now swollen and bleeding, put out a hand to Lucifer's chest.
"Sam, it's okay", Dean murmured through swollen lips. "It's okay. I'm here. I'm here. I'm not gonna leave you". Lucifer pounded him twice more. "I'm not gonna leave you".
Lucifer drew his fist back for another punch when sunlight glinted off the roof of the Impala, catching Lucifer's eye. His reflection was visible in the Impala's window. When he looked passed his reflection and through the window he could see the little green army man stuck in the ashtray. It was then that all of Sam's childhood memories of being in that car came bubbling to the surface.
Memories of young Sam stuffing the army man in the Impala's ashtray. Young Dean putting Legos in the Impala's vent. Young Sam and Dean carving their initials in the Impala. His mind flashed to when they were older. Thoughts of Sam going through Dean's cassette tapes while Dean filled the Impala's tank. Sam holding up the tape labeled "MOTORHEAD" to Dean while he sat in the driver's seat. DEAN taking the tape labeled "METALLICA" from Sam. So many other memories fly by him of the brothers in that car. Lucifer struggled to stay in control.
When the memories hit him of Dean handing Sam the keys to the Impala and hugging him, his fist unclenched. Sam panted and let go of his brother. Dean fell to the ground against the Impala. Sam backed away from him, struggling with Lucifer for control of his body.
"It's okay, Dean. It's gonna be okay. I've got him", Sam breathed. Sam took the Horseman's rings from his pocket and tossed them on the ground. "Bvtmon tabges babalon". He recited the incantation and a huge hole opened up in the earth. Sam moved close to the hole and looked down into the abyss. The abyss that would lead to the cage. The cage from which there was no return.
"Sam!", the Archangel Michael screamed. "It's not gonna end this way! Step back!" Air was being sucked into the caverness pit and both Sam and Michael were struggling to stay on their feet.
"You're gonna have to make me!", Sam yelled back. He stepped up to the hole ready to throw himself into it.
"I have to fight my brother, Sam!", Michael bellowed. "Here and now! It's my destiny!" Sam looked over at Dean, closed his eyes, and spread his arms out wide. Michael sprinted at Sam with his arms stretch out, to pull him away from the edge.
*At the last second Sam stepped to the side, sending Michael hurdling, head first, into the hole. Michael screamed as he fell and the hole closed up after him. The scream was cut off as the hole sealed itself. Dean looked at Sam with wide eyes. Sam looked back up at his brother and the grin that spread across his face, chilled Dean to the bone. Dean knew that the person standing there was NOT his brother. He was the creature. Lucifer walked over and picked up the rings that lay on the ground and shoved them back in his pocket and sighed as he walked back over to where Dean sat on the ground, leaning on the side of the Impala.
"I'm sorry Dean, but I'm just having too much fun here!", Lucifer purred. "You and Sam have kept me so entertained!" Dean sat on the cold ground, panting, his face covered in blood. A tear rolled down the side of Dean's face. He knew they had failed. Lucifer had Sam and there was nothing he could do about it.
"Please", Dean whispered. "Please don't take him away from me!" Lucifer pouted, kneeling down next to Dean. But the fake sadness didn't last more than a moment before Lucifer cracked a smile and snickered.
"You just don't get it, do you?" Lucifer laughed. "Sam is my perfect vessel!" He stood back up and looked down at Dean with a sneer. "Why would I give that up?" Dean dropped his head and stared at the ground. He couldn't believe this was happening. "Well, it's been fun Dean, but I've got things to do! People to torture!" Lucifer started to walk away, but turned back around abruptly. "But, you can be sure that I'll be back to play with you some more soon".
"LUCIFER! YOU GIVE MY BROTHER BACK! NOW!" Dean bellowed just before Lucifer disappear. "NO! SAMMY! NO! Dean crawled over to the spot where Sam had been standing not a moment before. He sat up on his knees and just stared up at the sky. "GOD! WHERE ARE YOU WHEN I NEED YOU?" He only received silence in return. Bobby and Cas were dead and his baby brother was being ridden by Lucifer like a two dollar whore. He felt utterly alone. In that moment he WAS alone.
"Sammy", Dean whispered. Hot tears rolled down his face as he grieved. Then He heard a flutter of wings next to him. When he looked over he saw Cas standing there with a look of sadness in his eyes. He knelt down next to Dean while Dean looked at him in shock.
"C-CAS?!", Dean sputtered. "You're alive! H-how?" Cas looked back at Dean with a kind smile.
"Yes, and I got an upgrade". Cas reached up and touched Dean on the forehead. His face was instantly healed and he stood up. "As for how, I believe GOD brought me back. New and improved". Cas walked over to Bobby and placed his hand on his forehead. Bright light glowed from Cas' hand, bringing Bobby back from the dead. Bobby opened his eyes and looked up at Cas with a puzzled look on his face. Cas helped him to his feet and they walked back over to Dean.
"What happened, Dean?", Bobby whispered. "Is Sam in the cage?" Dean looked away, running his hand over his mouth. After a long silence, Bobby got impatient. "Somebody tell me what the HELL is going on here!"
"Sam's not in the cage", Dean growled. "Michael and Adam are". Dean walked over to the Impala and leaned over the hood. "Lucifer disappeared wearing Sam like a cheap suit. I don't know where he went. But I will find him and get Sam back". Cas and Bobby walked over to Dean and they each placed a comforting hand on both of his shoulders.
They mourned with him in silence.