Author's Note: This takes place after season nine, so Dean is a demon. I meant for this to be a one shot, but then I started writing it out and it got a lot longer :)
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"Not being able to breathe is not very pleasant" thought Castiel as Dean's hand encircled his neck, trying to choke him.
They were in an abandoned warehouse. Sam and Cas had followed what they thought was a lost-and-confused Dean into the desolate building. That's when they found out it was a trap.
Cas's left hand was attempting, unsuccessfully, to pry the demon's grip off his windpipe. Meanwhile, Cas's right hand grasped Dean's left hand, keeping the First Blade from murdering the angel.
Dean's black eyes stared deep into Cas's blue ones, daring him to give up. "What's wrong?" Dean asked sarcastically, "demon got your tongue?" His mouth curled into a sideways grin. Cas could only let out another choked sound. His vision was blurring; blood from his head wound was trickling into his eyes making it worse. Dean looked as if he was a painting. 'By Van Gogh' some little part of Cas's brain randomly decided, 'that's what Dean looks like.' The rest of his brain, having decided what to do with his last ounce of strength, took his left hand away from the death grip and wrenched the First Blade from Dean's hand. The crazed grin was wiped off the demon's face as he released the suffocating angel and scrambled to the floor to pick up his precious.
But Cas had thought about this. As he fell, his vision a mess of colors that made the world resemble a Picasso more than a Van Gogh, his foot kick out, propelling the Blade across to the corner of the room, where Sam was stuck battling three demons.
As he lay in a heap on the floor, Cas could feel his grace attempting to heal him, as Dean had managed to also wound his shoulder earlier, but he pushed it back. He had an idea; a crazy, suicidal idea. An idea worthy of a Winchester.
Cas's oxygen-starved brain was spinning and he barely registered that Dean had run off towards Sam.
He mentally cursed himself. He had taken on Dean specifically so Sam didn't have to. If it came to it, Cas didn't think that Sam could kill his brother. The angel pulled himself off the ground and stumbled towards the brothers.
Sam had taken down the three demons with practiced ease. He whirled around to strike at the figure behind him, but froze and dropped his knives in shock when he saw who it was. "Dean." He said, not daring to let his voice sat more. The demon curled his mouth back into a grotesque parody of Dean's normal smile. "Hello Sammy." The light reflected off the shiny handle of the First Blade at Dean's feet. Sam quickly threw himself onto his brother, knocking the demon over. One punch hit Sam in the face, then another. Sam threw back one of his own. His fist collided with Dean's nose at the same time as Dean's fist collided with Sam's head, propelling the human into a heap on the warehouse floor. The demon hissed and reared back, his head smacking against the wall.
Cas, blood trickling down his forehead and right arm, collapsed next to the injured demon. Dean clutched his bleeding nose, and tried to pull himself up, only to slide back down the wall. "Concussion," a small part if Cas's brain diagnosed. He wiped away a trickle of blood and noticed that his hand was trembling. Not because he was scared but because he was tired. Cas found that his stolen grace was running out faster than he would have liked and it was effecting him both physically and mental. Cas was disturbed how human he sounded.
Cas glanced around. The warehouse looked darker than it should; He could barely make out Sam in the distance. Cas reached over to the demon, his hand still shaking, and touched his forehead.
He had thought that when the last of his (stolen) grace left him, he would feel... something. But there was only the endless feeling of being tired. So very tired. His hand dropped from Dean's forehead and into his lap, completely limp.
Cas watched as Dean's eyelids fluttered open. Even with his damaged and fading vision, Cas could see the emerald sparkle of Dean's eyes; the monstrous solid black had fled his beautiful eyes. Cas could no longer keep his own eyes open. "So beautiful," he murmured, his eyes shutting. " 'so beau... T-i...ff..."
The world was black. Cas hated that. The color black reminded him of Dean's eyes (dancing, sparkling, jovial eyes). Dean's eyes were green not black. Green like the wide expanse of forests that Father had made. How he missed Father.
A lone, grey figure appeared in the distance. It walked at a leisurely pace towards Cas until it was visible. Even though the world was still engulfed in darkness, Cas could see who the figure was: Death. Death, saying nothing, reached out his hand towards the fallen angel. Cas reached his own hand out instinctively.
The world exploded into a brilliant and blinding white light.