Dean knew how it all would end. In his heart he always knew it would end bloody, the way all hunters' lives ended. However, he thought the blood would be his and not that of his countless victims. Some deserved to die and some did not. However, it didn't matter because when he came into contact with the living he killed it.
For he was dead. He had been that way for a long time. Since, since he killed...
Not that it matter, his past did not matter, there was no past, present or future. He knew only death and blood.
Cas had been right, Dean turned into a monster. Though it did not take centuries as he predicted, it happened a few months after Charlie's death. He was still human when he turned. The experience taught him that being evil was a choice. The black eyes were mere byproducts of that choice, a final reminder of his decent into darkness. He turned his back on humanity, seeking to end it rather than embrace it.
He spent years, as Cas put it, murdering the world. Most days he could not remember who he had been, some days the distant memories were like echoes mocking him for not acknowledging the past.
When he became a demon again he continued the cycle of booze, women and murder. Killing became his only release. This pattern went on for years, centuries, until one day for a reason he cannot mention, he prayed or maybe it was just a memory of a prayer similar to those he recited in purgatory.
"Cas," he thought, "I don't even know if you're alive. Hell, I don't even know if I'm alive. Sometimes I wish we could go back just you, me and S…"
He was interrupted by the fluttering of broken wings.
"Well I'll be dammed," Dean said while turning to face the intruder.
"Well technically you are," Cas proclaimed as he absentmindedly touched the side of his tattered trench coat. What once was a symbol of their bond had become a painful reminder that mocked him wherever he went.
Dean chuckled, "Haven't lost your sense of humor, what has it been 90 years? You look terrible. It's not just your meat suit, your grace seems, seems less…angelic."
Cas dropped his gaze, "Why did you call me here? If you want to kill me, I won't fight it," he almost whispered. He sounded sad and broken. He truly was a fallen angel, Dean thought.
Dean stepped closer, "Like you could even if you wanted to."
The broken angel averted his eyes as he uttered, "Just do it." It sounded almost like a prayer.
"I called to talk," Dean said gently.
Cas' head jerked up, a flicker of hope appeared in his eyes and stayed there until Dean spoke again.
"I wanted to know if you watched," Dean snarled. "Remember you said you'll be the one who watches me murder the world, so what do you think of my work?"
Cas cringed as he took in their surroundings. The ground was gray and red. Cas realized it was not dirt they stood on but rather old bones and flesh. A mass grave made by a mass murderer.
"I don't care," looking into Dean's eyes as he spoke, regaining courage he did not know he still possessed.
A look of confusion spread across Dean's face, "What?"
Cas' moved towards Dean, "I watched you be cruel but mostly you seemed lost. I wanted to hate you but I could never hate you. Kill me or torture me if you must, but still I would not hate you. Not even when you killed S..."
"Don't, spare me the chick flick moment. I called you because I was bored and needed someone to fight with since...well I sort of killed all my enemies," Dean chuckled.
After a moment Cas uttered one word, "Sam." Dean cringed as Cas went on, "His name was Sam. You might remember him as Sammy. You murdered him, you should never forget that." This was Cas' last shot to get through to Dean. He never thought he would get this chance again. He would save Dean or die trying.
For a moment Dean looked centuries younger, "Sammy." He whispered. "I did him a favor, his in Heaven now, isn't he?" he pleaded.
Cas silently nodded.
There was a hint of relief on Dean's face. It took only a moment for the mark to reclaim him. "It is you I cursed, killing you would've been a mercy."
Cas looked brokenhearted as he said, "I would give anything to save you."
"Listen blue eyes. I'm beyond salvation. So stop being a chatty Cathy and fight me." Dean was saying one thing but his eyes were saying another.
"My name is Castiel," Cas said deadpan.
Dean laughed, "Yea Cas, I figured you would keep the name, you know you're still the only Cas I've met." In that moment Cas saw a hint of the old Dean.
"I know why you called. You want me to kill you," Cas was merely trying to buy some time but Dean did not deny it.
"I can't die. Not even suicide works," Dean's dark tone indicated that he had tried this on more than one occasion.
In that moment, Dean's eyes showed an indication of the righteous man he was born to be. It was in that moment that Cas made up his mind. "Give me the mark," he looked in Dean's eyes trying to indicate that he was serious.
"What?" Dean asked, not willing to accept what his former angel was suggesting.
"Give me the mark and blade. I will kill you so you may go to Sam." Dean stood frozen feeling a host of emotions he never thought he would feel again. Cas put his hand on Dean's shoulder, "You taught me the meaning of sacrifice, of family and of love. Let me sacrifice myself for my family, please."
One tear slipped down Dean's face as he silently transferred the mark and the blade to Cas. He closed his eyes and braced himself for death and whatever was to come. "I'll see you in my Heaven, Cas."
Cas gently picked up Dean's body and buried it under an apple tree. Tears slipped down his cheeks as he faced the truth: he never cared about his own fate or that of the world. He only ever cared about Dean. The rest of the world could go to Hell.