Lucifer knows that if there's one thing Castiel is proud of, it's his wings. It's the one thing all angels are proud of, and the one thing Lucifer misses from his time in Heaven. Because they were beautiful, and they showed what you were, what you had earned, who you were to God himself, and Luci loves God. He loves God, and yet he was cast out, and this angel, this tiny, weak thing currently curled up in the bathtub of the rundown house, was rewarded twice over for rebelling against Heaven.
Luci knows Castiel can't stand the torture; he's balled himself up as tight as he possibly can, wings pressed to his back, furled and hidden against the pale skin of his host. He's bleeding from the mouth, and from one ear, and from scratches and welts down his back, and Lucifer still isn't finished. The only people who would help the angel are the Winchesters, and they don't know that Castiel isn't in Heaven. Instead, he's in a Hell all of his own, a Hell on Earth that Lucifer made for his baby brother.
Nick's body is rotting around him, and the sores littering his host's skin are obvious and painful, like mosquito bites, and Lucifer is getting annoyed with it, but he knows that after he does this, he can have his true vessel. Lucifer-in-Nick's-body kneels beside the blood-stained bath, leaning over and pressing his mouth to Cas' cheek, the angel shivering from the cold. Lucifer is like an ice cube, freezing cold and hypothermia-inducing, and even if Cas doesn't feel it, Jimmy reacts, shivers running up his spine.
Cas can't speak; Luci's all but crushed his throat, and although the angel will no doubt survive, he's choking on blood, his vocal chords bruised and unable to function. So Luci speaks for him, a hand trailing down his brother's back, ghosting over his dark wings. "You're a sinner, Castiel. Just like me. You rebelled. You defied your brothers. You defied our father."
Cas whimpered, the feeling of Lucifer's fingers against his wings foreign and upsetting; he hid them away for a reason. They weren't something to be manhandled like this. Lucifer clucks his tongue, kissing his brother's neck and pulling at his wings gently, attempting to coax them to unfurl. "Shh, Cas, come on… We're all each other has now. You might as well come with me willingly."
Blood drips down Castiel's chin and he stares as the crimson splatters onto the dingy porcelain of the bathtub, managing to choke out a barely audible, "No," before the blood coming out of his mouth turns black; it's the dead blood, the old blood, the blood that isn't supposed to come up unless you're close to death. Castiel can feel Jimmy inside his head, scratching away, and the feeling makes him nauseous; he hasn't felt this in months. He hasn't felt this close to dying in months.
Lucifer runs his thumb over Castiel's lips, blood slicking up his pale skin, and he pulls a face, kissing at his little brother's cheek in a mockery of love. "Castiel," he hums, nudging against his cheek gently. "I don't think you can hold out that much longer. You know why you feel so weak? Heaven's cut you off, Castiel. They know you're here with me, and they can't risk you spilling anything. They'd rather let you die than have you telling all their lovely secrets to me."
Tears burn at Castiel's eyes and he shakes his head, more blood rising up out of his throat, out of his lungs. "Faith," he whispers, choking on copper and iron and wishing, for a moment, that he really was dead. That this wasn't happening.
"Faith isn't going to get you out of this bathtub. And it's not going to heal you, and fix your neck, and make you all better," Luci murmurs, continuing to kiss his brother's skin. "They left you for the devil. And you have two choices, Castiel… you can either stay here, with me, willingly, or you can let me take it from you. Make it so you can't leave me."
Castiel shook his head slowly, a choking, bloody laugh leaving his lips as he glanced at Luci, looking disgusted. "No."
"Cas." It's obvious, though, that his little brother isn't going to listen, and Lucifer shrugs before wrapping a hand around the base of one of his wings, tugging at it slightly.
He guides Castiel into a sitting position, perching on the edge of the tub and drawing his brother in closer, a hand covering his little brother's eyes as he leans down, kissing him gently on the mouth. Cas' wings unfurl suddenly, stretching across the room, and Luci smiles a little, gripping one of his brother's wings tightly. "If I take these, you'll be with me. We can rebel together, like we were meant to."
Castiel goes limp, the tugging on his wings entirely unpleasant, and Luci decides to take his time, plucking the feathers from the eons-younger angel's wings, knowing each time one is taken out, his brother grows weaker, closer to breaking. Castiel is dry-heaving blood by the time Lucifer's finished with the first wing, and Luci moves his hand from Cas' eyes, looking him over cautiously. Castiel blinks up at him, blue eyes dull, and Lucifer steals a kiss, the motion reassuring and chaste. "I have to do this, Cas. I still love you the most."
Castiel knows for a fact that his brother is lying, and this is only reaffirmed when there's a solid tug on his wing. Lucifer gets his grip and then tears, pulling the wing out of Castiel slowly, making the agony twice as unbearable. Cas screams, God, does he ever scream, but Luci just smiles, freeing the wing from his back and dropping it on the floor of the tub, Castiel writhing in pain.
He does the same thing to Cas' remaining wing, leaving his little brother broken and bleeding in the bathtub, crimson pouring from the gaping holes in the angel's back. Luci slides into the bath beside him, cradling Castiel in his arms and whispering something about love, although Cas can't hear him through the pain currently seizing his brain. Luci smiles, kissing his brother on the temple and hugging him, movements affectionate and loving.
After all, he loves his little brother. He's no angel of mercy, but with Castiel, he can certainly try.