A/N: Loosely inspired by a piece of fanart, "Strange and Beautiful" by Haku-silk. Check it out if you can, it's very beautiful and worth all the time it takes!
I do not own Supernatural or any of its characters.
He was bleeding, but only barely aware of it. The wound was from a gunshot; it had hit him in the stomach, about the place where the liver would be, he thought, not fully knowing exactly what the body he used looked like inside. But he felt no pain. The pills, the absinth, and the fear still had possession of him. He thought he was dying and that didn't really matter either. It was darkening around the corners of his eyesight, dimly, shadowy.
There were dead bodies everywhere. All of them human. Some of them were the demons' former vessels, now empty of both demons and human souls. Some of them belonged to his friends. Except they were not friends anymore, but dead. There were still sounds of gunshots, of screams from the Crotes echoing on the large, almost empty planet. And he walked on, bleeding. Outdoors, into the garden.
There he found Dean.
He didn't make a sound. There was nothing to say. He just sat down beside Dean's lifeless body. Dead. Castiel put his hand on Dean's forehead but he could do no more than that. Once he could have saved him. Once he could have brought him back, but that was over long since. Dean was dead, and he was useless, as always since he became human. He couldn't do anything. He couldn't even move away. He didn't want to leave him here, although it was too late. He thought he would die sitting here, and he didn't care.
Dean was gone. The world was dead. He knew it was the end. He had known for a long time, and he had never been able to stop it from happening.
He didn't hear the angel approach. But he felt his touch, and the soft tingling down his spine. He looked down, to see where the wound had been and no longer was. Even the blood was gone.
"He's just a human," Lucifer said. "Just a human. What is the matter with you, brother?"
Castiel didn't answer. Lucifer went past him, past Dean, and then stood there, in front of him.
"Why are you still here?" Castiel asked. His voice was trembling. "You've got what you wanted. Why don't you leave?"
"Why would I want to?" Lucifer said. "This could be my home now. My world, the new paradise. It's purified; I cleaned it. Besides, where would I go? It's the same everywhere. To Heaven? I don't think they'd be exactly thrilled to have me back. And Hell…" He was quiet for a minute. "Well, I will never have to go back there again. Earth is my home now. My father's creation. That's almost like some sort of poetic justice, don't you think?"
Castiel looked up at him, and saw Sam Winchester looking back. Except of course it was not Sam Winchester. He could no longer see Lucifer's true form behind the bars of his human host, but he knew it was him. Lucifer's eyes, looking down on him through Sam's. Castiel remembered the day Sam had said yes. He remembered the pain he had seen in Dean then, the naked guilt. After that, Dean seemed to have stopped feeling anything. At least as far as he could see.
"It's over," Lucifer said and twisted Sam's face into a strange smile. "I've won."
"There are still people out there," Castiel said.
"Yes. Humans." The look of contempt didn't seem right in that face. "They will all die too. Either they will be killed, or they'll kill themselves, or old age will take them. It doesn't matter. They'll be gone and I'll be here. They can't stop me."
"I'm still here," Castiel said.
"And what are you, little brother? You're not an angel anymore. Not really human, either. A tattered and torn piece of angelic grace trapped behind human flesh. If I were to destroy your vessel, you'd be gone."
"Then do it," Castiel said. His hand was still resting on Dean's forehead. He couldn't muster the will to remove it. "Do it."
"No." The fallen angel crouched down before him. "I'm not going to kill you. I don't want to see you give up your life for this human. He's nothing but a puppet with his strings cut off. I won't let you cry for him."
"Why?" Castiel managed to say. "Why does it matter? You've killed others. Our brothers and sisters. Why not me?"
"We're the only one left here," Lucifer said simply. "And you're a lot like me. You've killed your brothers, just like me. I once asked you to join me, do you remember? The offer still stands. You don't have to be human anymore. There's nothing for you here. You're an angel, brother. Don't lose your pride."
"I-" He looked down at Dean. "I will never join you."
"You're being foolish, Castiel. It will end. And so will you, unless you let me save you."
"No. I don't want anything to do with you!"
"Suit yourself." He rose again. "And where will you go? Back to the humans? Knowing that you can no longer do anything for them? But you never cared about them, right? It was all about Dean." He started to walk. Castiel watched his legs as they passed him, and then Lucifer's voice came from behind him: "Too bad he never cared about you as much as you cared about him. He was going to sacrifice you. You were nothing but his shield. He threw you, you and all those miserable humans, into the butcher yard while he sneaked around the back. Trying to kill me. With a silly human weapon. He must have known it would never work. Look at him, Castiel. Look at the human you gave up everything for."
Castiel looked at him. He looked at Dean. Dean had called him his brother, the biggest honour he would ever give anyone. Dean had been there for him. When all the angels left, and he was still left on Earth and the pain was too big to bear, Dean had been the one to comfort him. Taking care of him the first time he was drunk, telling him the best way to throw up in a bucket, teaching him how to be human even though he barely seemed to know it himself. He had told Castiel that it didn't matter what you could and could not do, as long as you didn't give up, even if your sudden humanity was a little inconvenient. One just dealt with the problem as they came, he used to say. It could have been worse, because it fucking always would be eventually. But that had been before Sam had said yes. Then Dean had not been Dean anymore. And Castiel had not been able to help him. He never could have helped him. Saving him from Hell – only to have him ending up in an even bigger Hell, where there was no way out. He really was useless.
Now Lucifer was right behind him, and his hand – Sam's hand – touched Castiel's shoulder.
"Why did you do this to yourself?" he said, and Castiel flinched. "All this poison you're putting in yourself. That's a lowly human weakness. They poison themselves, and each other, and then they try to live like that. In filth and decay. You should not act like them. I can scourge you of all that, you know. Just say the word and I will heal you."
Castiel realized he was crying. He had never cried before. Jimmy had used to, sometimes, inside him, at the time when he could still feel Jimmy. But Jimmy was gone now, had disappeared into the void same as the angels.
He softly stroked Deans' face. It was starting to feel cooler, the way human bodies did when their heart stopped beating. No blood drifting around their veins anymore. Leaving them cold and slowly falling apart.
"You killed him," he said.
"Well, he was trying to do the same to me. It's a dog eat dog world out here, little brother. That's the way it's always been."
Castiel shook his head.
"You choose this, then?" Lucifer asked. "Instead of coming with me, you choose this?"
Castiel nodded, unable to speak anymore. He bent down to pick up Dean's body. It hung like a lifeless doll in his arms, too light for a human body. As if it was just an empty shell, and maybe it was. Then he started to walk. Away.
He heard the flutter of wings, and the next moment Lucifer was beside him.
"I've won," he said quietly. "The angels all fled with their tails between their legs. Mr Great Saviour of Mankind here failed. The world is broken, and it's mine. It's over. I won."
He softly touched Castiel's cheek. Again were the flutter of wings, and Lucifer was gone.