Hello all. I was feeling a little maudlin and decided to write this one shot. I hope you like it.

Disclaimer: I don't own Supernatural.

Castiel had stood watch over humanity a long time. Centuries in actual fact.

He had seen their creation, watched as they evolved from that first fish that pulled itself from the sea eons ago when he was standing at his brother's side, so young himself then. And yet it wasn't until he was sent into the deepest depths of hell to pull the righteous man from the clutches of the demons that he truly learnt anything about them. Learnt how they felt, how they were ruled by emotions, and how they always tried to fix what they thought was broken.

He had got it wrong when he had taken the first steps on his own road to what he believed was free will, hoping to make the angels more like the humans he had come to admire so much. He had thought he knew better than all others and as such had needed no council. But he had learnt from that mistake, in the most painful of ways. He had learnt that people fail, lose their way, can do the worst things possible for the right reasons. That the angels were no different to the humans he was trying to emulate. They just had more power, and as such there mistakes were more catastrophic.

He was old now, had seen so much more than any human could feasibly understand. Had been through so much more than he could ever express, and yet one thing he had learnt in his life stuck with him. It was the most important thing humanity had ever taught him, and it taught him it well, though it took some time for him to truly understand the lesson. And it was for that one piece of knowledge that he still stood there, watching over the world, walking among them, but seen by none, not really.

The thing he learnt, it was that the most important thing to being human, it was an enduring love and compassion for your family, and that family did not end in blood. Family was those closest to you, those who loved you despite the mistakes you had made, loved you no matter what you did.

His true family were dead and gone now, a long time past. They were up in heaven, or at least he liked to hope they were, he had not gone up there to look for them. It was best he kept himself away from the angels, they had still not forgiven him for all he had done to them, and he couldn't blame them for that. They weren't his true family after all. They would never forgive him, even if he had called them brother and sister for the whole of his existence. They would never understand the concept he had learnt at the Winchesters side. And while yes, he would agree that for a time that love for family had got a bit skewed, with Dean and Sam willing to die for each other, or allowing the world to burn just to save the other, but they got back to the true meaning in the end. Back to the place where they knew that if one of them died to save the world then the one left would honour their memory and let them rest in whatever peace they would have.

Of course that was probably more because it was Sam who was the one who survived. He was always the one who could and would live without his brother. But Dean had died heroically, as all hero's did, saving the world. Taking Amara with him as he went, with a smile on his face and a knife in hand. He died as he lived, with a fight, a crazy idea and for the moment.

He always said that he knew how he would die young, at the end of a gun or a blade, Castiel doubted though, that Dean could have ever seen that it would be his blade that ended him. He still wasn't sure he believed it himself. It was one of the reasons he stayed hidden. Even after all these years he wasn't sure he could face up to the fact that he had killed his best friend. Plunged his blade into his chest and held him as his life radiated out of him as it would have if he had been an angel. The only way to stop the darkness, to lock her back into the prison god had forced her into so long ago. The prison which required a sacrifice to lock.

God had given his favourite lieutenant, Lucifer, up the last time, giving up his goodness and light to surround her and hold her inside. And this time was no less painful a sacrifice, for who better than the righteous man to hold her? The one she had bound herself to. As long as Dean lived, then so would she. Sam hadn't seen it, or so he said, Castiel still wasn't sure about that one. But in that final fight, when Dean faced her once more and could do nothing but beg her to stop she gave him the key.

"We are bound, you and I. Together we can do more than you could possibly imagine, and we can be together forever. I can no more kill you than you can me. You are still the key Dean, still the one with the power to hold me. Do you not see that? We must be together."

And Dean had seen finally what he needed to do. And so he had asked him, his best friend to do what he knew neither he nor Sam could do. Take his life and close Amara back into her prison. Castiel hadn't wanted to do it. He had wanted to protect Dean, but he also knew that the world was more than just one man, and there were times when sacrifices needed to be made. And so he had plunged his blade into Dean's heart and watched as his soul wrapped around the woman who screamed out in pain, pulling her to away from that which Dean had given his life to protect.

The only thing Dean had asked of him in those last few moments of his life was to look after Sammy, make sure he didn't do anything stupid, and Castiel had, of course, agreed.

When it was done he had explained it to Sam, asking if he wanted him to stay, Sam had said no and so Castiel left but that didn't stop him watching the man. Watching him get over his grief for Dean and move on. Watching him keep on fighting and finding others to help and teach. It was when he knew Sam would be okay that he left. It was also then that he truly felt it for the first time. Going into a gas station rest room he stripped off his coat and opened his shirt so he could see. And yes, there it was. On his left shoulder a handprint was seared into his skin, identical to the one he had so long ago seared into Dean. It was then that he knew. He was the new keeper of the key to the darkness's prison. And it was a key he would protect for his eternity. Dean had given his life to lock her up, he would honour him by living to keep her there.

This was what family meant. Keeping the world safe while all those you loved left you because that was what they would want you to do. To keep on fighting to protect that which Dean had given his life for.

And so Castiel stood watch and waited. He watched as humanity died away as all such beings did at their time. He watched as the earth crumbled into the sun. And he watched as the universe started to implode, he knew then that it was time. Pulling out his blade he held it over his own heart and plunged it in deep. With a burst of light his grace left him and he died, the mark on his shoulder dissolving into nothingness. Suddenly there was a rush of force across the space of the universe as Amara once again escaped her prison, but this time she wasn't alone, she had beside her her caretaker, the one who had guarded her for so long, making sure that none of her essence slipped out to the key as it had done the last time. She had been right, they were destined to spend entirety together.

For at her side was Dean Winchester, and now his job was done he had only one thought in his head. To find his family. He rushed across the dying universe to the place that the earth had once stood and that was where he found him. Castiel dead with a serene smile upon his face. He had kept his human body all this time and Dean found himself smiling at the familiar face. Moving as only the essence he now was could, he picked up Cas' body and held it to him as the angel had done for him while he died. And he whispered into the dead angels ear.

"We did it Cas. We saved the world."

With that he closed his eyes and let himself be taken wherever it was he would go. He didn't care anymore. He was done. His job was done. He could rest now.

"Dean open your eyes. Your home." With that Dean blinked, that voice, he hadn't heard it in so long, it couldn't be, could it? Opening his eyes Dean found himself in the bunker looking up at his brother who was smiling at him with tears in his eyes.

"Sammy." Dean replied with a smile raising his hand to his neck so he could confirm he was really there. It was then he heard a commotion, and looking round he saw that all the people he had loved in his life were there with him. Bobby, his mom and dad, Ellen and Jo. And there standing with his hands in his pockets was Cas.

"Your right Sammy, I am home. Home with my family." He said as he stared at the angel who had given up just as much as he had to save the world. He had no idea what it would cost Cas when he had asked him to kill him. But he had done it. Cas had saved the world just as much as he had.

And Castiel just smiled back and nodded. Dean was right, they were both back home where they belonged. With their true family who loved them for who they were, no matter what.

It truly was the only important thing he had ever learnt over those billions of years standing watch, to love and be loved by his family.