I can do this, Sam thinks at first, I can.

He hasn't spoken to Dean in going on a year when he teams up with Ellen and Jo and manages to take down Famine [with some aid of demon blood. He doesn't tell anyone that] and Pestilence [they lose Jo in that fight. Then Ellen. By then it's just Sam-all-alone again]. They've split their forces down the middle; Sam knows Bobby and Castiel are with Dean and now he has no one.

Sam hangs himself on a bridge to greet Death. They argue. Sam tells Death he plans to take Lucifer in just to stop the disease of the devil from spreading. I'm not strong enough, Sam says, But if I kill myself when he's inside me, maybe I can win. Death tells him he's foolish and arrogant if he thinks he can kill himself when an archangel festering in his gut. That's when Sam finds out about the rings, the key, the Cage.

That's when Sam writes a note to Dean, via Bobby, drives to Detroit on a drunk man's suicide mission, and says yes. He brings the small band of hunters he's gathered, the few stupid enough to follow him. Strangers whose names or faces he might've known, a lifetime ago. They go to war, demons against humans, a bloodbath that stains the air with salty copper. Sam drags himself through the carnage to fall at Lucifer's feet.

He says yes.

Sam isn't strong enough. He can't do this. Not alone. There's no one to pull him back from the edge, no strength to lean into when hope is gone.

It starts with the Croatoan virus, it starts with what have I done, it starts with a quaint cottage Lucifer builds behind Sam's eyes. He takes Sam there and makes tea for both of them and hand-feeds Sam his own intestines on a stick when Sam has to watch the Croat feeding on Bobby's skin; paralyzed Bobby, gun gone, helpless to defend himself. Watches Dean kneel in front of the wheelchair and grab Bobby by the back of his neck and hug him close. Watches Dean put the gun to Bobby's gut and pull the trigger five times.

That's when Lucifer comes out. That's when he steps out of hiding, in Sam's body, in Sam's jacket and boots and Sam's skin but it's not Sam, not really. Dean turns away from Bobby's corpse and just stares and stares, and Sam is terrified because there's no warmth in Sam's eyes. No nothing.

"What happened to you?" Dean asks, bracingly, and Sam realizes from the house in his head that he's probably changed. He's not what Dean remembers. His features might be more like a predator's and his cheekbones might cut and his eyes cut deeper.

"Detroit happened to me." Lucifer spreads his arms wide, a taunt, a dare. "MFEO, Dean. Me and this one."

Dean puts the gun to Sam's head, and pulls the trigger.

Lucifer disappears.

It becomes a game of cat and mouse, Dean chasing him across the country. Sam tries to beat his way free but there's never enough strength, never enough time. Sometimes they watch Dean form the shadows, watch as he turns harder and harder like a diamond shell over a bleeding wound, all that thick scarlet sludge trapped on the other side. Dean bleeds out into his own insides.

Sam screams, every life they take. He screams out loud, every blow Lucifer absorbs, and kills the hunter who crossed paths with him. Dean, I'm here, I'm right here, see me, hear me, help me, Dean, please!

The walls of the house start to rot but they're still just as strong. Lucifer doesn't return to the cage inside Sam's head where Sam is hostage. He rules the world with an iron fist. No smiles. No laughter. No music. Just demons everywhere, their stench in Sam's nostrils, his own helpless pleas making him deaf as he batters to break free and save his brother save the world. He knows what Lucifer is waiting for.

Sam watches Dean kill a man with his bare hands. No remorse. Just resolve. Watches the darkest part of John Winchester they always feared, manifesting in spades across Dean's emotionless, pitiless face and his relentless ways.

The atmosphere changes. Something shifts.

The next day, the hunters attack.

Just a ragtag team. Half a dozen soldiers at most. Lucifer leaves the demons in charge and descends to the back garden and that's where Dean finds him; gun in hand, eyes like a dead man's, two graves green as moss, and lifeless. "Found you, you son of a bitch."

"I was never really running, Dean. You just didn't want to see me." Lucifer raises Sam's hand, outstretched. "You gave me this world, Dean. Because you weren't there. You didn't understand the secret. Overlooked it. It's pathetic."

"You killed Sam five years ago," Dean's voice is flat, no inflection around the words. Like he's speaking about a dog that was hit by a car, or a favorite gun that Lucifer stole from him. "You took my brother."

"We've been over this. He gave himself. Willingly. He knew you'd given up on him, so what was left? Sam put the pieces together."

No, Dean, that's not true, I did this for you, I was trying to save us all—Dean, look at me!

"Dean! Look at me!" Sam's voice erupts, desperate, from Lucifer's mouth, and for just a second Dean's defenses fall away. His finger bounces off the trigger. His eyes widen. Sam sees his mouth curl to one side.

"S—Sammy?"

Lucifer has him on the ground in a second, one foot pinning Dean's throat. "Fooled you."

No, no, Dean! Oh God, you can't do this, you swore, you promised you wouldn't hurt—

Lucifer's head twitches at another sound, a swish of wet leaves, and with a jerk of his ankle the light goes out of Dean's eyes. He goes limp, with Sam's name the last word to leave his mouth.

And then Lucifer turns, and Dean is behind them. A different Dean. Less rough edges and more vulnerability at the sight of Sam's face, Sam's shape. And before the knowledge spirals through Lucifer and into Sam, Sam already knows this is his Dean, somehow.

"Oh. Hello, Dean."

Touch him, and I swear to God I'll kill you.

Lucifer chuckles, inside the headspace they share. Well. Aren't you a surprise.