Sam remembered once speaking to Jimmy, Castiel's vessel when he was himself again, and hearing him say that being taken over by an angel was similar to being chained to a comet.
Sam had been amazed, at first, but soon realized how harrowing being connected to an angel for so long would really be. Never free, blinded by the overwhelming power of a celestial being as it slowly consumed you, your life gradually being leached away, your time stolen by an angel.
Sam also remembered being possessed by a demon, holding a knife to Jo Harvelle's throat, a creature using his own voice to ask his brother to kill him. He had no control, his body, for those moments, not truly his own.
Being taken over by Lucifer was different. It wasn't like being attached to an unending rocket of light, or like struggling to regain control of your body. It was like being locked in a dark room, unable to move, unable to breathe, but the whole time you can see, you can hear, you can feel. And you can think. You listen to this being laugh at you as you repent over and over letting him in, as you curse him again and again, until you're simply alone with your own regrets and thoughts and hatred that slowly festers.
Alone. Completely alone.
Sam had never felt so alone and helpless—not when Dean had been sent to hell and he had looked everywhere for a way to bring him back to no avail, not when he had been in the panic room going through detox, screaming for help but none coming, not all those times when Dean had had to protect him for harm and injured himself in the process.
But the worst part was not the time he spent under Lucifer's control as the apocalypse raged all around him. He'd said yes because he thought he could use it to stop the devil. But he'd been wrong. He didn't know how to stop he apocalypse, he didn't know how to stop Lucifer, and he didn't know where his brother was.
That was what he regretted the most. That was the worst part—leaving his brother behind in this screwed-up world, never able to make things right with him. Never to see him again, probably. He could have been dead for all Sam knew. He wished more than anything that, if he couldn't stop what was happening, then at least he could take his own life and join Dean on the other side, wherever that might be.
The day Sam finally saw his brother again, Lucifer knew that they were coming, knew they were on their way to kill him, and was not concerned. "Don't worry," he told Sam. "They don't pose a threat, not anymore. I always knew this was coming. You and I will be spending quite a bit more time together."
Sam did not respond.
"There's a group of them on their way, I hear. Castiel is with them, that pathetic fallen angel. Your brother Dean, too, I believe."
Sam flinched at the sound of his brother's name. Lucifer chuckled. "Oh, that got your attention, didn't it? You miss him, don't you, Sammy boy? Too bad he'll just die like the rest, the moment he faces me. Or, I'm sorry…faces you."
Go to hell, Sam muttered.
Lucifer laughed again. "It's too bad, really. You haven't seen him in, what—three years? And now he's walking right into his death. It's all right, I promise to make it quick."
Sam wished Lucifer was lying, that his brother wasn't coming, even that he was already dead. At least then he wouldn't have to see him die, or worse, watch Lucifer kill him with his own hands.
He should have known that luck would not be on his side.
Lucifer waited in the garden as the sound of gunshots and screaming slowly began to surround them. Sam tried to listen for a familiar voice but heard none, only death and destruction.
Lucifer smiled with Sam's mouth when he heard footsteps approaching behind him at a run. He turned slowly and Sam, trapped within his own body, let out a cry of anguish. Dean stood less than six feet away, weapon in hand, eyes widening in dismay as Lucifer smiled at him. His face was hardened, Sam noticed, aged and bitterly determined and furious. He'd seen that look before, when they were hunting, but now the lines of hatred seemed permanently etched into his brother's face.
Do it! Sam pleaded silently. Please, do it now! He could see the pain in his brother's eyes, the recognition that it was already too late, that he couldn't kill Lucifer, that he probably never would have been able to.
"Welcome, Dean Winchester," Lucifer said softly. "I'm very sorry."
Dean opened his mouth to reply, but Lucifer stretched out a hand before he could speak, flinging Dean to the ground. Dean tried to get up, stubbornly continuing to try and fight, but Lucifer approached his side and pressed down on his neck with one foot. He gazed down dispassionately at Dean, while Dean stared back, his eyes filled with, of all things, apology.
Please, Sam said. Please don't.
"Too late," Lucifer said quietly.
"Sammy," Dean whispered, and Sam felt that word like a punch in the chest, more painful than anything Lucifer could inflict on him. After all this time, all he could ever do was disappoint his big brother. And yet, after all the mistakes he'd made, he was still Sammy. He was still Dean's little brother, and Dean's last action was to reach out and try to find him amidst all of Lucifer's fire.
It was futile, of course. Sam struggled against it with every force of his will, but it wasn't enough. It was never enough. He felt Lucifer press down and twist his foot, and he felt Dean's neck snap.
Sam let out a strangled scream of pain and anger, a scream that went unheard and unfelt except to Lucifer, who only laughed inside Sam's head. Hurts, doesn't it, Sammy? Well, it's about to get worse.
Just as he was wondering what Lucifer meant, he lifted his head from Sam's dead brother and turned slowly. And there, standing at the other end of the garden, was Dean.
Except Dean was still lying at Lucifer's feet. This Dean, Sam realized, was different. Younger. He was staring at Lucifer with a mixture of pain and disgust, but Lucifer didn't move to kill him like he had before. He greeted him calmly, while Sam beat furiously, pointlessly, at the walls of his prison, shouting at his brother to run, to move, to go somewhere else, anywhere else. Sam would not watch his brother die twice in one day. Not after all this time of wanting to simply see his face, hear his familiar, comforting voice, apologize for everything he'd done.
"Oh," Lucifer said, as though pleasantly surprised. "Hello, Dean."
"You've come a long way to see this, haven't you?"
Dean whirled around towards where Lucifer had transported himself, the look of pure fear flitting across Dean's face as he backed away ripping through Sam like knives. His brother, Dean, was afraid of him—no, of Lucifer. He couldn't let himself forget that.
Dean's jaw worked. His face had relaxed into a hard mask of pure hatred. "Well, go ahead," he said. "Kill me."
Lucifer tilted his head to one side in confusion. "Kill you?" He glanced at the other Dean, lying motionless in the grass. "Don't you think that would be a little…redundant?"
Dean glanced at his other self and then back at Lucifer, anger flashing in his eyes. Sam shouted at him to run, to get to safety so he could live, dammit—but of course Dean couldn't hear. Because Sam was trapped, chained for eternity.
Lucifer sighed at Dean's lack of reaction. "I'm sorry," he said without much sincerity. He began to step forward slowly, approaching Dean. "It must be painful, speaking to me in this…shape."
Dean only gazed at Lucifer coldly as he continued to approach. Sam could see the familiar green of his brother's eyes, could count the freckles adorning his nose. He was so damn close, if only he could reach him, speak with him…
"But it had to be your brother." There was a sincere note of apology in Lucifer's voice now. "It had to be."
Sam's heart leapt as Lucifer reached for Dean's shoulder, a split second sob of relief escaping his broken spirit. After so long and so much distance, and he could finally touch his brother, even if just for a moment, even if just one hand, he'd never dreamed he'd get to contact his brother again—
But no. Sam saw the moment that Dean jerked away from Lucifer's hand in slow motion, Dean looking at him as though he were something vile, repulsive, eyes flashing in a furious warning, and Lucifer lowered his arm. And meanwhile Sam was inside screaming, tearing at his bonds, shouting at Lucifer, at his brother. Dean, please! I'm still in here, it's me, Sam! Let me touch him! Just let him, Dean, just for a second, please! DEAN!
Lucifer flinched, for the first time seeming to acknowledge Sam's presence, but the moment Sam thought he had gained some headway Lucifer forced him back, tightening the bonds he had enchained Sam with. And Sam gave up trying to call out to his brother with another broken sob, knowing it was pointless.
He was already too late.