"When you finally turn – and you will turn – Sam, and everyone you know, everyone you love, they could be long dead. Everyone except me. I'm the one who will have to watch you murder the world."

A one-shot based off of this quote from 10x22 The Prisoner.

Castiel was old. Very old. He had seen things that humans could never imagine.

He had watched with mild interest, the rise and fall of empires, and not just human ones. At one point it was plant life who ruled the Earth. Then there were the dinosaurs.

And then came the humans, and Castiel fell in love with them. He fell in love with humanity. They were far from perfect, of course, but they were pure. They had good hearts. And their souls shinned brighter than any angel grace. And so he watched them.

For centuries he simply watched from Heaven. Although he wanted to make the trip down the Earth, he wanted to walk among the strange but magnificent creatures, he knew it was not his right to do so. It was God's will that he stay where he was, and let them be.

But then God left. Castiel didn't know it at first. He knew things were changing in Heaven, but he knew better than to question his orders. Another century went by without God and his guidance, and things on Earth were falling apart. And as much as it pained Castiel to see it, he stayed put.

And then, when Castiel wasn't paying attention, Dean Winchester was born. There didn't seem to be anything extraordinary about him, and Castiel never glanced in his direction. He watched others while this boy's life was torn apart, and pasted back together as something knew. He followed orders as this little boy grew up, loosing so much and sacrificing the rest. It was only when he made the deal that everything changed.

Special orders came to Castiel. They weren't only for him. He was but one of many soldiers called to make the trek to Hell. It was a dangerous mission, but one that Castiel accepted with zeal. It was always an honor to do God's work.

Out of all the angels called, it was Castiel who rescued Dean Winchester. When he reached him, Castiel had his doubts. This man he was rescuing, did he deserve to be saved? Castiel watched him torture souls with glee in his green eyes. Did he want to be saved? But once again, Castiel reminded himself that it was God who commanded this soul be rescued, so he did as he was told.

The moment that Castiel laid his hands on this man, this boy, really, was the moment that would change the course of Castiel's life. For eons, the angel had done little changing. He simply was. But now, everything was different.

His time with Dean Winchester proved to not only be the hardest years of his life, but the best. It was this man who taught him what it meant to be kind. What it meant to care, to think for yourself. What it meant to be human.

But the thing was, Castiel was not human. And soon, neither was Dean.

Dean Winchester had been through a lot of shit in his life, and much of it Castiel witnessed himself. But the Mark of Cain proved to be more than Dean could bear. It overtook him, it became him. And Castiel knew, everyone knew, that this half-human half-demon walking around in Dean's skin, using Dean's voice, killing with Dean's hands, was not actually Dean. The Dean Winchester that Castiel had come to know, the boy who lost everything and still gave so much, who watched out for his brother, who saved the world when all odds were against him, was dead. And he was replaced with this evil thing masquerading around as his friend. He knew this, but it didn't make a difference.

Dean was strong. He held out for a long time. He kept himself from tipping over the edge into full fledged monster. And as the years went by, he pulled farther and farther away from family and friends. Castiel kept an eye on him the best he could, but he didn't bother him. It took Sam a long time to adjust, and even then he never truly accepted it. He missed his brother, and Castiel understood the feeling. Sam became reckless. He rode around with no real purpose, hunting and killing and trying to keep his mind off his brother. Sometimes he would pray to Castiel, and Castiel would come to his side, but mostly they stayed apart.

And eventually the day came, that everyone knew would come. Although he had sworn to keep his distance, Castiel told Dean the news about his brother. He told him how he died, wondering if it would incite any kind of emotion from Dean.

Dean said nothing, but Castiel swore that as he turned his back, he could see a tear.

Time went faster after that. Centuries went by, and again, Castiel found himself watching from above. He was no longer among the humans he loved so dearly, he couldn't bear to be. He watched them all fondly, but paid special attention to Dean.

Castiel watched Dean's struggle, and wanted desperately to help. But he had made it very clear that he didn't want his help. The man kept to himself for many years, but both of them knew this delicate balance would not last forever.

"Maybe you could fight the Mark for years, maybe centuries like Cain did, but you cannot fight it forever. And when you finally turn – and you will turn – Sam, and everyone you know, everyone you love, they could be long dead. Everyone except me. I'm the one who will have to watch you murder the world. So if there's even a small chance that we can save you, I won't let you walk out of this room."

And sure enough, just as Castiel predicted, Dean snapped.

Castiel wanted to stop him. He knew he had to. He couldn't just let Dean tear through these humans like tissue paper. Dean—the real Dean—wouldn't want this. He would want to be stopped, no matter what.

So Castiel tracked him down. He found him in a house not his own. Castiel went from room to room, finding the family who lived there, brutally murdered in their beds. Dean was in the living room, blood plastered across his face and hands, soaking into his clothes. Castiel stood across the room, facing him. "You can't do this, Dean. You have to stop."

"I can't stop, Cas." Dean stared him down, gripping a knife in one hand. He looked down at it, a smile creeping onto his face. "I don't want to."

"I'll have to stop you." Castiel let his angel blade fall out of his sleeve and into the palm of his hand. "I can't let you do this."

"Maybe not, but you will." Dean replied, knowingly. His black eyes met Castiel's, and it made the angel shiver. "You won't kill me, Castiel. You can't. You're sentimental, you're weak."

Castiel recalled back to the time he was being controlled by Naomi. Even with practice, he couldn't kill Dean. "I've had centuries to consider this, Dean. Centuries to prepare."

"It won't be enough. No amount of time will. Because I remember. I am far past feeling emotional, far past caring, but it doesn't mean I don't remember what it's like to feel." Dean took a few steps toward Castiel, who did nothing. "I remember what it was like to care about you, to want what's best for you. And I know that back then, I never could have killed you." He paused, for dramatic effect. "I'd rather have died myself. And as much as you are an angel, you're no better than I was. You love me, Castiel. As a friend, as a brother, as more, I don't know. And I no longer care. But I know you won't kill me."

Castiel listened to his words and knew he was right. He gripped the angel blade, willing himself to lunge. It's not Dean. It's not Dean. But it was. It was his voice and his face and his eyes. It was his chest he would be shoving his blade into. It would be his blood that poured out onto the floor. And even then, it would take so much more than that to truly kill him. He wasn't sure he was up to the challenge. "Why don't you just kill me, if you care so little. You're more than capable."

Dean held up his knife again, twirling it in his fingers. "I'm not interested in killing you, Castiel. You said it yourself. You were going to watch me murder the world. Well, here we are. You and me, buddy, until the end of time. Watch me."

Before Castiel could say anything more, Dean was already moving. Castiel tried to stop him, but Dean threw him into the wall with ease. He fell through the dry wall, and by the time he was back up, Dean was gone.

Castiel never gave up on him. He would track Dean down and try to stop him, but he could never kill him. He argued with him, he begged him to stop, to listen. But nothing worked. It was all simply a game to Dean. He would allow Castiel to come near him, to talk to him, and then just push him away. He could've killed the angel long ago, but that would be no fun.

They kept this up for years, Castiel wasn't even sure how long. He had stopped counting long ago.

Millennium's past by. They remained constant in a world that changed drastically around them. Humans came and humans left. Generations past, and species died out. Castiel paid little attention to their failures and their accomplishments. Eventually, they themselves died out on planet Earth. The species would live on in other galaxies, but never here again.

But others would come to replace them. A new species would rise from the ashes, believing themselves to be the first, just as humans did. And the whole process will start again, but it would be different.

Castiel found Dean, once more. It had been a long time since he had last talked to him. The ex-hunter was walking alone in the forest, his blade dragging along the tree trunks. Castiel wondered if he, too, pondered what would happen to the Earth, now that the humans were gone. Now that his kind was gone, if he could still call himself that. "Dean."

Dean stopped walking, but he didn't turn around to see the angel. When he spoke, it was different from the other thousands of times they had met. He had been so hostile, so cocky. But now, while he still didn't sound as he once did, he greeted Castiel more like an old friend than foe. "Castiel. How long has it been? A couple decades this time? You're slacking, Angel."

"I've been watching you. You've been out here, alone, for months." Castiel didn't dare move any closer, even though he wanted to see Dean's expression.

Dean laughed in a way that surprised Castiel. "Well, that's just creepy."

"What are you doing out here, Dean?"

Dean turned to the angel. Castiel was shocked to see the exhaustion in his face. His eyes were black, but when Dean blinked, they flicked back to green. He was smirking, but the smile didn't reach his eyes. He took the knife and stabbed it into the tree, without even looking at it. "I'm tired, Cas." Castiel didn't say anything. He had no idea what to say. "How do the angels do this? How do they deal with this... Immortality? It's exhausting."

"We were built for it. Humans, on the other hand, were not. It was always going to wear you down."

"Took it long enough." Dean muttered, glancing down at his arm. Although his sleeve covered it, there was no mistaking that he was looking at the Mark of Cain. The smile left his face, and what replaced it was grim and slightly unnerving to the angel. "I've been this way a long time, Cas. A long time."

"I know."

"Everyone I've known, everyone I once cared about, is dead. Except you."

"We both knew it would be so."

Dean let out a long sigh. "Can I ask you something, Cas?"

"Always."

"Do you think it would have worked? The Book of the Damned? Do you think it would have cured me?"

Castiel was surprised by the question, but answered anyway. "I don't know... I believe it would have, but the consequences would have been great. Possibly greater than one should be willing to pay."

"But you would have. Wouldn't you?" Dean asked, squinting his eyes, curiously. "You and Sam would have done whatever it took."

Castiel paused. "Yes. If you had let us."

"You're a good man, Castiel. Although you aren't one. That's why I need your help."

"With what?"

"I want you to kill me."

Castiel's eyes widened. His mouth opened, but no words came out. He couldn't be serious. After all these years, all these chances to accept his help, Dean waited until now to reach out. What had changed? "I don't understand."

"It's pretty simple. You should have a long time ago. A really fucking long time ago." Dean leaned against a tree and folded his arms. He looked down at Castiel with tired eyes. "I've been alive a long time, and like you said, I ain't fit for it. I'm tired. I'm ready."

"What's changed your mind?"

Dean shrugged. "I have no idea. I've had a lot of time to think. And this is what I have decided."

Castiel shook his head. "I'm not going to kill you."

Dean rolled his eyes and pushed himself back into standing position. "You're going to pussy out on me again?" He replied angrily. "I'm asking for this, Castiel. I'm asking. And you know me, I don't ask for anything. Not anymore."

"But Dean, listen to yourself! Listen to the choices you're making!" Castiel argued back. He took a risk and grabbed Dean's arm, the one with the Mark. "The Mark doesn't want you to die, Dean. So if you're making the conscience decision to do so, it means the Mark isn't controlling you. Not as much, anyway."

Dean pulled his hand away. "You don't understand, Cas. I can still feel it. I can feel the lure of it. It's calling to me. Begging me to kill again. Begging me to kill you."

"But you're not listening to it. You're fighting. That means something."

"You think?" Dean asked, quietly.

"I know."

Dean laughed, and it reminded Castiel of the old Dean. It had been so many years, so many decades, so many centuries since he had seen even the slightest hint of that Dean. He never thought he'd see it again, although he had always hoped.

All this time, he had held that image in his mind; the image of the Dean who convinced him to go against Heaven, and do the right thing. The Dean who believed in his innocence, even when all signs pointed to his guilt. The Dean who kept his trench coat when he waded into that pond, praying that he would come back. The Dean who had stood by his side for many years. Maybe, after all this time, there was a chance that that Dean would return.

"I've done a lot of bad, Castiel. More bad than I can ever make up for. Why would you even want to help me?"

Castiel frowned. It was a good question, one that he didn't think he could honestly answer. Dean was right – while he had done a lot of good in his human life, it would not out way the bad he did afterwords. The people he slaughtered and tortured, they greatly outnumbered the people he had saved. So why would an angel, who had devoted his life to helping God's creations, help someone like that? "Have you ever read the Bible, Dean? All the way through?"

"What does that have to do with anything?"

"Have you?"

"Not cover to cover... but yes."

"My Father has taken many lives, Dean. He sent a flood to wipe out all life on Earth but just a few. He has killed nonbelievers, and killed first born's just because of their parents actions. And the angels, such as myself? They are warriors. They kill, many times with little remorse, for what they believe is God's will. And me? Although I have had only the best intentions, I have done the same.

I don't agree with that. With the angels, nor my Father. I don't believe that we should kill those who disagree with us, or those who anger us. But even the best of us fall in these traps. What you've done, Dean, is horrible. I am not sure that there is anyone who can forgive you. But then again, I don't know who could forgive any of us. I'm not saying you deserve my help, but I am willing to give it to you."

Dean thought for a moment, and the two stood in silence. Finally, he nodded. "Thank you, Castiel."

Castiel smiled, for the first time in a long time. This was probably a bad idea. Dean was dangerous. And although he seemed at ease now, he couldn't fight the Mark forever. He had already proven that. And yet, Castiel was willing, once again, to trust him, to believe in him. Because at the end of the day, Dean was always his weakness.