Regardless of what Dean seems to think, demon blood doesn't leave the system after a night's worth of screaming in solitary. From Sam's experience, it never has and never will. Just because he can put a leash on it doesn't make it tame. It walks in front of him and tests the strength of his grip when it pulls, but that's not what Sam's afraid of. He's scared of the day that it realizes it has teeth, and that the leash only keeps it from running away, not turning around.
But he makes it through most nights without even a growl. Or ignores them as best he can when they froth up from his veins, his stomach, his spinning head. Nights where he knocks himself cold with exertion or gnaws on his fingers to the point of bleeding. He doesn't know if he can drug himself with his own blood, and he doesn't want to risk it. Iron and sulfur gets spit into the sink, and his mouth gets washed out with a rigor that leaves his gums feeling raw. Dean regards him with equal parts worry and pride, offering a beer for the taste.
Sam's gotten better at hiding his extracurriculars. Better at making sure they're a need, not an indulgence, and he would know indulgence well, given how much of his life has been one. Stanford and Ruby and now, Lucifer on the end of his bed, legs crossed. He's decided he hates shoes, and so Sam has to contend with the bewildering mundanity that is the devil having toes.
Not that they're his. Sam tries not to think about that.
They have an arrangement: Lucifer travels the world he hasn't seen in centuries until he can prove that the bad outweighs the good, and Sam doesn't recoil from him when he wants to talk.
"Where'd you go today?" Sam asks. He runs his tongue over his upper molars when he shuts his mouth. He knows they should taste like mint after brushing his teeth. They don't.
"I have a bridge up in Wales I went to look at," Lucifer says, conversationally. Sam's not sure what contributes more to his pleasant moods, an excuse to wander the world or to show up on Sam's doorstep whenever he wants. Sam's never had anything to go back to while traveling his entire life. Lucifer's is newly acquired, and he never leaves Sam for so long that he gets homesick. Sam listens to him carry on. "Well. Not mine, exactly, but if they say I built it and there's no one left alive to dispute the story, then it might as well be."
Sam snorts. Lucifer tilts his head. There's no confusion in it, like there might be if Sam was laughing at Cas's blunt statements. Lucifer makes jokes on purpose. He makes jokes to see what works on Sam and what to lean into. Sam's not sure how comfortable he is with Lucifer molding himself to better fit him.
The thing is: the arrangement is supposed to be one of those paradoxes. Counting every grain of sand on a beach. Shaving a mountain down with a nail file. Weighing up every last stone on earth against every sin of humanity. Those kinds of things were impossible when performed by mortal men. Lucifer's infinite, and Sam has no way of checking his progress.
Getting Sam to say 'yes' isn't off the table. It might only be on hold. In the meantime, Lucifer insinuates himself in every facet of Sam's life he can find because it never occurs to him that there isn't space left over for him to occupy.
"Is that all it takes? Someone slaps your name on a bridge, and you take ownership?" Sam asks. Lucifer considers that.
"I don't see why not."
"We've got a lot of those. Devil's punchbowls and canyons and waterfalls." Lucifer knows this already. He has a fixation on humanity's fixation on him. Narcissism might be the easy way to explain it, but sometimes Sam thinks Lucifer's that desperate to be seen. Sam dreads the day he finds out about the Edlund novels. (Sam has read them all, so there he is, one for one with Lucifer on obsession.)
"Is that what you wanted to spend the night talking about?" Lucifer sounds almost hopeful. He wants to be called on for late night conversations. What he gets is Sam praying to him for one reason and one reason only. The need part of this whole arrangement. Lucifer still comes running. Sam works his jaw.
"We had a hunt that went bad."
"Are you hurt?" Lucifer asks.
"I'm fine," Sam answers. Lucifer doesn't believe him. He has good reason to. Last time he asked, Sam lied boldly with a black eye adorning his face.
As if him praying wasn't enough of a tip off.
"They tied me up," Sam continues, "and Dean was unconscious. They were going to sacrifice him. Tear his guts out and scatter them across the altar like-" Sam doesn't say. Doesn't have to. Lucifer was around for the creation of hellhounds. He knows what they do to a body when they want the soul inside. "Another hunter got us out, but I couldn't do anything. I was completely powerless." There it is, the fact that has his stomach rolling and his hands shaking, bad as any night where he missed a dose. Nothing was easier to tempt with than the strength to protect himself, to protect Dean. Lucifer knows better than anyone why Sam finds power so attractive. He's the walking, talking embodiment of what Sam's terrified to become, lashing out violently at anyone who dares to get in his way or think about locking him away again. He's also the only one Sam can turn to. Castiel could provide, maybe, but he's Dean's angel, first and foremost, and if his brother ever caught wind of this... Sam's not sure he'd be let back out of the panic room.
Sam can't remember much of the comedown after Famine, but he thinks there was a moment between the fear and the vomiting up of every ounce of blood in his guts where he was laid down and a cold hand pressed against his forehead. Dream or not, it had been the only point of comfort through the entire ordeal.
Lucifer takes him in, and then he begins to undress. It's methodical and quick. Sam's mouth waters despite himself, and he swallows.
When he's done, Lucifer spreads his hands like an invitation. The body is not his, but the body isn't what's important. Sam joins him on the bed. He's shaking. He can't say if that's entirely anticipation. Lucifer lifts a hand to his face and gently traces it, thumb pressing between his brows until the furrow there smooths, fingertips sliding along the sleepless shadows under his eyes, touching his lips. Sam's mouth falls open on instinct, and he reprimands himself with a wince. Lucifer frowns, but he doesn't say anything, pulling his hand away. He brings his wrist to his own mouth. Ruby liked the clean slice of a knife through her borrowed flesh. Lucifer bites his skin open with his teeth. The smell of his blood hits Sam like a freight train, and he doesn't think before he grabs at Lucifer's arm and drags it close.
He only manages to stop himself with his mouth hovering an inch from the bleeding wound. He tries to take back control. Lucifer innocuously shifts, pressing his wrist to Sam's mouth, and Sam loses the battle with himself instantly. Lucifer's blood tastes nothing like a demon's. It's sharp, and cold, and Sam's soul welcomes it like his body did Ruby's. His mouth fits perfectly over the gash Lucifer's teeth made. He sucks, tongue pressing around the wound like he learned to do with Ruby when he wanted to coax out another drop or two. It's unnecessary with Lucifer. Ruby kept his doses strict, but Lucifer wants to give him everything. Enough that Sam could burn with it.
Sam hates and loves in equal measure when Lucifer's other hand comes up to stroke his hair. There's too much sense-memory there, and he half expects Ruby's voice in his ear, praising him, but when Lucifer speaks, it's unmistakably him. He keeps his voice light, teasing, "Someone's hungry, huh, Sam?"
It's like the first time he got drunk, and the first time he held a gun, and the first time he felt Lucifer in that church, all that horrifying, familiar power that fit where Sam's life had broken him open. It's like drinking lightning and feeling it weave through the lichtenberg figure of his veins. Sam's senses are dull these days, completely human, but Lucifer blows them wide open. And when Sam looks at him, he shines.
"More?" he asks. Sam laps at his wrist imploringly
Lucifer's blood won't leave him shivering and empty when the power fades again. It's not an invasive species threatening Sam with its unkempt teeth, but a keystone predator setting the whole ecosystem back into place. It drips steadily down Sam's throat. This is why he has to hope Lucifer's task really is impossible, because without even making him say yes, Lucifer's found his way into his bones and organs, and he feels right. Is Sam supposed to deny it when Lucifer's already been inside him? Sam turns from Lucifer's bleeding wrist and touches his chest. He pushes, and Lucifer slides down to the sheets. "Can I?" Sam begs. "Lucifer, can I?"
He doesn't want to say it. The only words Sam has to describe what he wants would dirty a sacred act.
"Take and eat your fill, Sammy," Lucifer answers. Sam pounces hungrily. It's one thing to lick at an opened wound, another entirely to feel his teeth sink into the muscle of Lucifer's chest. His mouth floods with blood as he bears down. Sam twists and jerks his head, grinding his teeth until the flesh gives. He swallows it, raw and red, and a piece of Lucifer settles deep inside him. Lucifer himself smiles, fingers coming up to sweep through the blood on Sam's mouth. He collects the morsels of his own flesh that escaped Sam's hunger and pushes them into his mouth. No one should be so happy about being devoured alive. Sam bows and digs his teeth in again. Lucifer feels no pain, or else doesn't show it, and the only sounds he makes are words of encouragement as he's torn up.
Sam wants to kiss him. Lucifer would let him, no matter how much viscera covers his face. What's blood to an archangel? Lucifer probably used to dip his wings in primordial ooze when his dad wasn't looking, like a kid eating the sand out of the sandbox when they're left alone. The lines between need and indulgence blur. Sam focuses on what he knows, that only Lucifer's flesh and blood tame the angry thing that demon blood left behind, and so he eats. The intoxication after Ruby was heady and dark, power that made Sam's head spin, but Lucifer fills him with crystal clarity. The only similarity is that no matter how much he has of either, it never feels like enough.
"This is my body," Lucifer murmurs as Sam licks his bones clean of blood, only for more to flood his mouth in the crevice he's torn open, "and this is my blood of our covenant. What should I promise? Forgiveness of sins?"
Whose, Sam wants to ask, but then his hunger overwhelms him. Lucifer still carries the taste of purity, which is more than Sam has ever been able to say about himself.
Sam should see the signs of an advancing addiction. They started these nights with Lucifer offering him a few burning drops. That used to make Sam's head spin so bad he'd have trouble standing. Now, he consumes strips of Lucifer's borrowed flesh easily. Lucifer reaches down to his own torso and pulls it open for Sam, fingers hooked in skin and muscle. The corners of his lungs poke from below his ribcage, expanding and contracting with air Lucifer doesn't need. Sam's eyes trace up and without thinking, he uses his nails to pull the gash wider. The bright white of Lucifer's sternum sits protectively over his beating heart, and Sam hungers.
"Whoever eats my flesh," Lucifer says, as he lets go of the gaping hole in his torso (It doesn't close up, keeping his insides exposed to Sam's enthralled gaze) and reaches up to cup Sam's face. Bloody hands meet a bloody jaw, washed with sin by gluttony, smearing it into Sam's pores like being guzzled down by him isn't enough, "and drinks my blood has eternal life, and I will raise him up at the last day. You will be glorious."
Lucifer will consume him from the inside. Sam will swallow him whole. The snake eats its own tail.
Sam agreed to this because he couldn't sleep another night suffering from withdrawal symptoms. Lucifer leaves none in his wake. Lucifer does not withdraw. Only slumbers, waiting to be rejoined as his whole self. Every bite lets him in, and it's only because he wants to do it right, wants Sam to say yes to him in every way, that he doesn't make himself a home in Sam's guts and ride out the end of the world there.
And Sam keeps coming to him. Sam is so fucking hungry.
He pulls back, dizzy with the beat of Lucifer's being that pervades his stomach, seeping out into every part of him. Lucifer follows him, sitting up with the cavernous maw of his torso, organs dangling like uvula. Sam shuts his eyes and tries to breathe through the iron clinging to the inside of his mouth. "Enough," he says, "no more." Lucifer leans into him, resting his chin on Sam's shoulder. Sam shivers, wondering if he's stealing from a memory of Jess or if it's an impulse all the devil's own.
"Do you want me to clean up?" Lucifer asks. It's casual again. Like Sam isn't wiggling his tongue against the gaps in his teeth trying to get a stray sinew free.
"Please." He can be polite if Lucifer can be a feast. Lucifer turns his head, mouth brushing Sam's neck. He doesn't kiss him, and Sam aches for it despite himself. Lucifer's grace settles over him and the space around them like a heavy fog. Sam breathes it in. It lifts as quickly as it fell, and with it, all the viscera and gore that turned the bed into a living nightmare and exposed Sam's grotesque desires is gone. Sam slides his tongue over his teeth. The blood is gone. They still don't taste like mint.
Lucifer is inside him. He digests slowly.
"Until next time?" Lucifer is playing with Sam's hair, absently carding his fingers through it. Not quite like a lover. More like someone would set their own straight in the morning after a long night. Sam licks his lips.
"I'll call on you," he says.