Based on a writing prompt. Prompt will be posted below.

Dean gasped for air as he tried to cling onto life long enough to get everything out that he wanted to tell Sam. While his brother desperately tried to do something - anything - to help, Castiel stood in a dark corner of the shed, invisible to the human eye, and watched the scene that was unravelling in front of him.
It was odd. The things one remembered when watching someone die. Things that had not mattered in the grand scheme of things but now came crushing down on the angel. Dean Winchester. The Righteous Man. The vessel of archangel Michael. The human who spat destiny in the face and helped to kill God. He who beat death itself - many times.
That very man was now dying his last death. Killed on a hunt like any other. An accident that could have been prevented if he had just stepped a bit further to the side. And Castiel remembered all the times that Dean had told him how he would die on a hunt. That he would never grow old.
The angel had never really believed that.
Dean Winchester was too great. Too skilled as a hunter. Meant too much to this universe. To him.
As it turned out, he was wrong. Dean was a human after all. A special one, that much was undeniable, but still a human. And humans burned for a brief time. Even if they burned like Dean Winchester, as bright as a supernova. Though, Castiel suspected, that might be the reason why his time on earth had been cut so short. The fire had been too big and hot for the candle of age.
Sam allowed Dean to go, while Castiel could do nothing but watch as the life was fading from the human he had fallen for - in every sense of the word. It was unfair to him, that this man had sacrificed so much for this world, over and over again, and yet he had been denied to enjoy more than a few weeks of the peaceful life that he had fought for. It was given to him, after all these years, and then immediately ripped from under his feet.
Dean closed his eyes for the last time. Memories of evenings with shared beers, seemingly endless battles in purgatory, and discussions over 70s rock bands flooded through Castiel's mind, and he had to avert his gaze for a moment, to remind himself that this was what life was supposed to be. No deals, no imbalance. Death meant death.
This was what they had been fighting for.
When he looked back up, a reaper had already appeared. It took the form of a young woman, dark hair that resembled that of Amara and brown eyes. Their name was Serena. Dean followed willingly, though not before taking a last look at Sam, who was trying not to break down completely. He did not see Cas - the angel had disguised himself from his eyes. He would only get in contact with Dean when his soul had safely settled into heaven and digested the reality of his death. Serena, however, saw him. The reaper gave him a nod, warm and knowing.
When they were gone, Castiel once more turned his gaze towards Sam and the body of Dean. The younger brother still held onto him, crying. After a moment of hesitation, the angel walked over to him - still hidden. He carefully touched two fingers against the man's temple, unbeknownst to him, and slowly Sam calmed down. Not entirely, because angels could not affect emotions like that, but enough to take a deep, shaky breath and finally pull Dean's body from the rod. Enough to take care of the vampire corpses and enough to drive back to their motel, to get Miracle and prepare the hunter's funeral.
When Castiel stood beside him, invisible to the human eye, and watched the flames consume the body of the man he had pulled from the pits of hell and stitched together, he heard his voice. Praying to him from heaven. And all the angel could do was follow. Like he always did.