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“Can you really beat her?” Castiel asks.
Lucifer pauses, eyebrows lifting. “I can.”
“Then yes,” Castiel rumbles.
Lucifer’s satisfied smile is swallowed in the blinding flash of light that follows.
~~
At first, Castiel and Lucifer don’t both fit. There is pushing and shoving, a lot of grumbling and hisses of pain as Lucifer’s wings flare out in his new space and poke Castiel in the eye, a cry when one of the wicked hooks on the end of Lucifer’s wing catches Cas across the face and leaves a deep cut that leaks grace down his chin. Lucifer twists himself into the shape of his vessel, straining against its other occupant as they collide and are forced together again in the limited space. Castiel is shoved back against the edges of the confinement, forced to tighten into himself as Lucifer takes up too much space like always, presence huge and hard and barbed.
“Sorry, little brother,” Lucifer laughs after a moment, combing through his own wings and fixing the ruffled white feathers. “I didn’t know it was gonna be this tight a fit.” He gazes at Castiel with a smirk firmly in place, his eyes gleaming with malevolent joy. “Hope you’re not too cramped,” he gloats, and expands a bit more, his edges digging painfully into Castiel’s grace, and the seraph draws yet further from Lucifer. “Now that I’m in charge of this meatbag, things are gonna run a little different around here.”
Castiel’s throat is tight when he speaks. “That is… fine , brother,” he says coolly, “but know that you are only here to defeat the Darkness. After that, the Winchesters and I are going to put you back in the cage.”
Lucifer laughs once, sharply, as his eyes narrow and he fixes Cas with an icy look. “Really now,” he drawls nonchalantly, and pauses for an instant. Then the smirk is back as he says, “You always were tenacious, brother. I think that’s why God liked you so much. You never gave up, even in the face of overwhelming odds.”
In a blink Lucifer crowds into Castiel’s space, pins him down with his staggering strength and crushes him in a vice-like grip with his grace. The archangel holds Castiel’s chin between his fingers almost gently, though, in direct opposition to the ruthless way he binds his sibling, and lets his fingers stroke along the seraph’s jaw. He heals Castiel’s split skin easily, without a thought.
“But this is nothing like that,” Lucifer purrs. “I own you from the inside out, and there isn’t a thing you can do about it. Yes, I’ll destroy the Darkness for you; she’s as much a threat to you as she is to me. But after that, you think you can stop me? Me? I killed you once already, Castiel, with a snap of my fingers. I could do the same to Sam and Dean with less effort.”
Castiel’s frown deepens and he pulls the corner of his mouth back into a sneer, holding Lucifer’s gaze and glaring back defiantly. “We did it once before. We can do it again,” Cas snarls.
Lucifer’s smile slides across his face sharp as a razor blade. “But things are different this time, Cassie. I have you. Do you think your little friends would kill me if it meant killing you too? No, they’re too soft for that now.”
Castiel flinches almost imperceptibly in Lucifer’s hold, but it doesn’t escape the archangel’s notice. Lucifer’s brow furrows briefly and he cocks his head. Then slowly it dawns on him, and his smile spits into a wide grin. “You think they would, don’t you? You think the Winchesters would really kill their pet angel?” Castiel says nothing, but his eyes flash dark and angry.
Lucifer straightens and takes a few steps back, humming to himself. “Well then little brother, we have to make sure we become invaluable, don’t we. Too important to kill. What’s the best way to do that?” He looks at Castiel and pauses, as if waiting for the seraph to contribute his two cents. Castiel is cold and silent. “No ideas? That’s fine, I’ll think of something.” Lucifer drums his fingers on his chin for a moment before his face lights up. “I know! We’ll start with Dean. There always was something between you two, and I bet with a little love and nurturing, we can grow it into something great.”
Castiel breaks his silence with a growl, and Lucifer can feel him straining against the confines of the grace that holds him. “Don’t you dare touch Dean!” he barks, face positively livid.
Lucifer simply smiles again and clicks his tongue like chiding a naughty child. “Oh, Cassie. I’ll do what you never had the courage to do. I promise, I’ll even let you get some enjoyment out of it. Everything you ever wanted, I can achieve it, and you’re lucky enough to still be along for the ride.”
Lucifer steps close again and runs his fingers through Castiel’s dark hair like a mother soothing her son, but it is wrong when Lucifer does it, mocking and cruel. They stare at each other, Cas’s chest heaving, gaze unflinching. “Oh, Cassie,” the archangel repeats. “Just watch me.”