I don't own anything, yada yada yada. Posted over on my LiveJournal. General Season 5 spoliers.
Enjoy!
Castiel can remember, with absolute clarity, the moment he lost what was left of his Grace. Dean Winchester's face was torn with pain, his eyes wet with fear, and his mouth – the same mouth that had formed over Castiel's just moments before – formed over the word "No." Dean's voice was lost next to the Heavenly Host currently singing its swan song in Castiel's ears. He listened in earnest, realizing this might be the last time he hears the song of Heaven and truly understands its words. His Grace was full and white as it left him and for the first time it was too bright for his eyes. He shut them on instinct. The Grace left him in a rush and it burned like a knife to the gut and stung like his heart was on fire.
But he did not scream.
This was a worthy sacrifice, Dean's life would be saved – and therefore the World. Castiel was at peace with that idea. He felt his entire being becoming smaller, tighter, restricted. He closed his eyes, preparing for the free Fall – the hard landing and the humanity that would come after that.
With his last wisp of Grace, he prayed his plan would work, that he had saved everyone; that Dean would live a long and happy life.
When he opened his eyes again he was momentarily confused. He expected for the confusion to leave him – he recognized it, acknowledged it, but it did not pass. This was new. The confusion took hold of his entire being and then denial joined in before he could process it. He was supposed to Fall. He was supposed to be reborn. Lucifer was supposed to take his Grace and let Dean and the others leave.
Looking up, confusion still clouding his mind, Castiel saw Lucifer glowing, his vessel looking healthier than he had in a long time. Lucifer smiled through the glow; my Grace, Castiel thought with a deep feeling of longing (he even took a step forward with an arm reached out towards the edges of the glow as if he could take it back until fear ran through him when Lucifer opened his eyes stopping him cold in his place.)
"Why thank you brother," he looked down at his hands and then stretched his shoulders all the way through his neck. "A dose of Grace was just what the doctor ordered."
"You son of a bitch," Castiel heard somewhere to his right. He tried to turn towards the voice, but found himself becoming dizzy. His hand reached up to steady his head, the tips of his fingers cold as ice and sending shivers through his skin.
"…what?" Castiel tried to speak. He had so many questions. Why didn't he Fall? Why was he still here? Would Dean still be free? Was Lucifer going to go back on his word? He wanted to ask them all, but his throat seemed tighter and suddenly he was racked with coughing.
Someone called his name, but it barely registered as the ground was suddenly getting closer. This must be it. He put out his hands to catch himself. I must finally be Falling. The feeling of his hands hitting the rough concrete of the old factory was new – a dull pressure that moved all the way up into his shoulders before his elbows gave out.
His head hit the ground with a bounce – he remembers because he thought it was funny how his vessel's head bounced like it was a play toy and how he thought that it shouldn't be that funny. He saw hands – familiar and warm – and then he saw Dean's face.
He felt warmth flutter through him as he saw Dean's mouth – the same one that kissed him, the one that was causing Castiel to become very distracted from this whole 'he should be Falling business' – form his name Cas like it was prayer over and over again.
And then his vision slowly became black.