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He's barely angel, mostly human when Lucifer finds him lying in a hospital.
Lucifer is there to see Pestilence's handiwork, and to see human suffering. (It's the sort of fascination young boys have for frying ants on the sidewalk and watching them squirms, really.)
To find an angel, no matter how weak, in one of these places? Lucifer admits curiosity as he strides into the room and sits next to Castiel. (not much surprise there)
"You're dying," Lucifer says.
Castiel nods, itchy and in pain. He knows he hasn't got much time left, regardless of whether the Winchesters succeed or not. Human lives are short, and he is barely an angel.
"My offer's still open." Lucifer spreads his hands. "You could survive this."
"I won't." Castiel says. He believes he won't. Lucifer can see despair in Castiel, and that? That makes him lean forward, interested.
"I don't understand you, Castiel," Lucifer says. "The Winchesters won't win, and if I die you'll die soon after."
"Dean didn't say yes." Castiel says, and he closes his eyes. He ignores Lucifer.
Lucifer is silent for a moment, taking in the sight of helpless Castiel, human Castiel. He's looking at such a weak creature who would dare to ignore him, and who would choose to place such faith in an even weaker human.
Lucifer is silent, contemplating his options. His offer will never be accepted, regardless of how he changes it, he can see that.
And he wants Castiel by his side. Castiel was just one foot soldier, barely worth a mention before he met the Winchesters.
Now he is stronger than any angel in some ways, nearly human, and Lucifer wonders what it would be like to have Castiel under his command, powerful again by Lucifer's will.
Oh, he would be cursed, Lucifer knows, but that's nothing new. And Castiel would be worth it.
Castiel isn't dead and cleared off of the stage, like he should be, and for that alone Lucifer is nearly willing to claim him for his own.
So Lucifer speaks. "I won't let you die, Castiel. Not yet."
Castiel's eyes snap open and he looks at Lucifer, just in time to see Lucifer cutting one of his wrists.
(There are other ways to share power in the way Lucifer wants to share power, but blood has always been potent. A primal display of power, and as human as it is Lucifer likes it.)
It is easy to force several drops down Castiel's throat.
It is easier still to destroy Castiel's cellphone when it rings, and then to pull Castiel to his feet, away from the bed.
Lucifer catches Castiel in another bloody kiss, once they are standing, and he does not let Castiel pull away, even when Castiel uses his unwanted strength.
"You're one of mine, now," Lucifer hisses, and Castiel looks him in the eye.
Castiel looks the Devil in the eye and says: "You will never have anything more than my body."
And Lucifer believes him.