I mean, Cuthbert said he was going to make Sam suffer unimaginably so what exactly does that entail?

Maybe he severs Sam’s Achilles tendons.

Maybe he carves Sam’s eye out of its socket and feeds it to Dean.

Perhaps he breaks Sam’s wrists and ankles with a sledgehammer.

God forbid he slices off Sam’s nipples.

I sure hope he doesn’t strangle Sam to the point of turning blue on the edge of passing out before releasing him, allowing him to catch his breath just to start the whole thing over again.

All the while Dean has a front row seat to it all where he remains untouched but he’s writhing as if he’s the one being tortured. Listening to Sam’s screams of agony. Seeing him covered in blood, hanging on by a thread. Dean can’t take it. He closes his eyes. He looks away. But he can’t help but hear the gurgling in the back of Sam’s throat as he chokes on his own blood.

Dean tries to scream at Magnus to stop but his voice is hoarse from doing so for the past 18 hours. Sam’s voice is long gone. All he can do is whimper and groan. Dean promises he’ll do whatever Magnus wants if he just lets Sam go but Sam is making such pretty noises that Cuthbert can’t help himself. He keeps going.

Sam’s breathing is raspy after Cuthbert wailed on him with a metal baseball bat. He hit him in the ribs (breaking several), balls, jaw and kneecaps. Sam’s jaw was definitely broken and it sounded like he had a punctured lung based on his gasps and breathing. He’s passed out from the pain and Dean would assume he’s dead if it weren’t for his terrifying, wet, and uneven gasps for air.

And Magnus leaves them like that while he goes to take a rest. Sam’s now on the floor completely unrestrained because it’s not like Sam can go anywhere.

After trying so hard not to watch Sam’s torture, suddenly, Dean can’t look away as he watches his little brother struggle for air on the floor. He’s begging Sam to hang on, keep breathing, wake up, but Sam doesn’t stir. Sam doesn’t hear Dean’s apologies cuz he feels like he’s underwater. Sam doesn’t have much fight left and he lets go. Stops trying so damn hard to breathe and just embraces the darkness.

Dean watches Sam’s chest rise and fall and rise and fall and then stop. Dean is screaming and pulling harder than ever at his restraints but it’s too late. Sam is gone.

When Magnus returns, Dean has tears running down his face.

“You said you weren’t gonna kill him!” Dean spits bitterly. Magnus looks down at Sam’s body and makes a “tsk” noise.

“Shame. Don’t blame me, Dean. I didn’t kill him. It’s the punctured lung that did it. Or the shock. Hard to say. But you’re quite right. Our fun can’t end here.” Dean watches horrified as Magnus assembles ingredients for a spell. He blows a handful of hand mixed powder into Sam’s face and says words in a language Dean can’t quite place. All of a sudden, Sam inhales a gasping breath followed by a horrid, bloody coughing fit. Once Sam stops coughing, he’s groaning in agony once more.

“What the hell did you do?” Dean asks, horrified as Sam writhes in pain.

“Well, I couldn’t have Sam tapping out early, now, could I? It’s a fairly simple spell with some not so simple ingredients but it does the trick. Learned that one from a necromancer. What I like about this spell is that, not only does it bring Sam back, it locks him in his body as-is. I couldn’t kill him if I tried. Well, not without removing his head and burning every piece of him. Now, the downside is that his wounds will not heal. He still feels every bit of pain. And since he’s died, he’ll now start to decompose, all while his brain remains very much alive and aware. It’s remarkable, isn’t it? I’ve just given your brother immortality, Dean. How does that feel?”

Dean stares at Magnus in horror, processing what he just said. He looks to Sam who is still delirious with pain, clearly not following the conversation.

“You son of a bitch,” Dean says weakly. Dean feels deflated, defeated. He never imagined his brother being forced to suffer this fate far worse than death. Guilt clutches him in his throat and he can’t breathe. Cuthbert smiles and turns back towards Sam.

“Now, where were we?” He grins at Sam while brandishing a scalpel.

Dean watches horrified as the torture continues and eventually Sam realizes something is very wrong with him.

“Dean? Why can’t I die? I-I should be dead. Why-why am I not dead? What did you do this time?” Sam pleads desperately and Dean doesn’t know how to tell him what’s wrong. He feels guilty as hell for all the decisions he made that brought them to this point.

I'm going nuts looking for this fanfic that I read ages ago. It's set at the end of season two where Dean sells his soul to bring Sam back. Sam's brought back but it's just his soul in his dead body. So Sam's body is rotting away bit by bit and he's loosing feeling and sense of self. It's a horror fic. One part towards the end he goes blind, eyes milk white and he can't speak so he's sitting on Bobby's porch tapping out a plea in morse code for Dean to kill him or something like that. it was a very good fic and I simply can't find it anywhere.

I've tried 'zombie Sam', 'undead Sam' etc on ao3. Fanfic.net isn't easy to search if you don't remember the author or title. I think I read it before I was an ao3 member. Its not in my history (I've searched thru the 1000+fics I read)