Set somewhere in season 5.
"Dean!" screamed Sam, falling to his knees at his brother's side. "Dean!"
Dean did not react. He just lay there, blood pouring from the gaping wound in his chest. They might have killed the demon, but in falling it had taken its revenge.
"Dean, come on. Dean! Don't do this to me, man." Sam looked around franticly, for something, anything, that might help save his brother. He could call an ambulance, but even if they weren't in the middle of nowhere, by the time any help arrived Dean would have long ago bled out.
"Dean, please. Hang on, Dean, just hang on."
"Sammy," whispered the older Winchester.
"I'm here, Dean. I'm here."
"Sammy." Dean's hand twitched as if trying to reach for his brother.
"I'm here," said Sam, taking Dean's hand and fighting back tears.
"Sammy, I- I- I'm sorry." Dean's eyes were sliding shut.
"No. No! Dean!"
Sam did not hear the flutter of wings behind him. Castiel had seen the laughing demon fading away and had been flitting through the area in search of the reason for its laughter.
"Move, Sam."
"Cas?"
"Move."
Sam reluctantly stepped away from his dying brother and Castiel knelt beside Dean.
"Knife," he said, holding out his hand.
"What?"
"Do you want me to save your brother or not?"
Sam wordlessly handed Castiel Ruby's knife.
The angel cut away the cloth around Dean's wound. He handed the weapon back to Sam and began to mutter spells in Enochian.
Sam watched over his brother's body as Castiel's hands and mouth began to glow while the rest of his body grew pale and somehow weak looking. As the light grew more and more intense, Sam had to look away, but he could hear Castiel still muttering in the angelic tongue.
"Sam," he said, raising blue eyes to Dean's brother.
"Yeah?"
"I think I'll be able to save him."
"Oh thank God."
"However, I won't survive it."
"What? No, Cas, we need you."
"No. Dean is far more important," said Castiel, speaking quickly. "Please tell Dean that I cared for him very much and that I'm sorry I couldn't help you two more."
"But Cas-."
"Tell him, Sam. Promise me."
"I will. Of course I will."
"Thank you."
Castiel bent low over Dean, pressing his hands to the hole in the human's chest and nearly touching Dean's lips with his. Still whispering Enochian magic, Castiel took all of his focused angelic power, from his grace to the energy spent keeping his vessel alive, and forced into Dean's still-warm body.
There was a burst of light and wind that nearly knocked Sam off his feet and Castiel's power flowed into Dean. His skin knit shut and angelic air filled his lungs.
Dean gasped, life back in his body, and Castiel crumpled to the ground.
Dean looked up and saw Sam, who quickly knelt beside him.
"Sammy? What happened?"
"Are you Okay?"
Dean slowly raised a hand to his chest and felt flawless skin beneath his tattered, blood-soaked shirt.
"I'm fine," he said, confused. "What happened?"
"Cas."
"Cas?" Dean followed his brother's gaze to the body lying beside him. "Cas!" Dean rolled over and half sat up to get a better look at his fallen friend. "What happened to him?"
"He saved you."
"Is he gunna be Okay?"
Sam was silent.
"Sammy?"
"I think he gave you his life."
"What do you mean?" asked Dean slowly, dreading the answer.
"He's- He's dead, Dean."
Dean stared at the angel's body in horror.
"No," he whispered. "No, Cas, no!"
Dean held a shaking hand in front of the mouth and nose he knew as Castiel's and felt nothing. Jimmy's body was empty and growing cold.
"Why?" whispered Dean, biting his lip and feeling tears stinging his eyes.
"He said you were more important."
"Cas."
"He wanted me to tell you something."
"What?" asked Dean, raising green eyes shining with tears to his brother.
"He wanted you to know that he really cared about you and that he was sorry he couldn't help us more."
Dean shut his eyes tightly, willing himself not to cry.
"I'm gunna go check on the Impala," said Sam, wanting to give his brother some space.
"The Impala?" If something had happened to his baby too-.
"She's fine, Dean. Just thought you'd like some space."
"Yeah," he replied thickly.
Sam turned and walked away, leaving his brother kneeling beside their friend's empty body.
Dean stared at Jimmy's discarded form, not knowing what to do or think. Finally, he began to speak, in a rough, shaky voice.
"Cas? Cas, I don't know if you can hear me or if you're even still anywhere; you never did tell us what happens to angels when they die, but, well, thank you. I wish you hadn't done it. I'm not more important than you, but thanks. I suppose someone's gotta look after Sammy, huh?" He ran a shaking hand over his face and then pressed his fist to his mouth, still fighting to suppress tears. "I really cared about you too, Cas, and I just hope you knew that. I don't know where you are now, but I hope you're somewhere and I hope its somewhere nice. Maybe angels get their own heaven? If you are in Heaven, I guess I won't be seeing you there when it's all over, but you never know, right?" Dean could feel himself breaking as it truly hit him that he would never see Castiel again. "Oh Cas, I'm sorry. I'm so, so sorry. A lot of good that does, I know, but what else can I say?" He was crying now, hot tears coursing down his cheeks, his shoulders shaking. "I'm sorry, Cas, I'm sorry." Dean raised his eyes to the heavens. The clouds on the western horizon were stained blood red and the first stars were just starting to show above him. Was Cas somewhere up there or was he just gone?