It has been five years since Lucifer rose. Five years of Hell on earth and Dean was responsible for it...well not directly of course but Dean is the kind of person who held all the blame upon his shoulders. Yeah, he is that kind of person. But enough of Dean and his "guilt/martyr" complex let us go on to the story.
A month after the Devil rose, Dean and his brother Sam split up: Sam did not trust himself with the hunt, he was still struggling with his addiction to demon blood, their last job proved he could not be around the temptation. Dean surprised Sam by agreeing with him. He thought they were better hunters when they were apart.
The brothers parted ways, each of their destinations unknown.
Sam found an honest job: he was a bartender, while Dean continued hunting on his own, trying to find a way to kill the Devil without becoming the Archangel Michael's vessel. Well, Dean was not on his own per se; he had the renegade angel Castiel with him. They made an unlikely duo, but they were efficient in dispatching demons and angels alike. They had a bond neither wanted to admit to, for fear the other would no longer be there, either by being killed or worse.
Two years after Lucifer rose, Dean and Castiel were still kicking butt. (A year and a half earlier Sam called Dean to inform him he was Lucifer's chosen vessel and had asked Dean to rejoin him and Castiel in the hunt, but Dean said no, saying they were stronger when they were apart. Dean hung up after saying this and has not heard from Sam since.) During that year, a virus erupted all over the world, a demonic virus that Dean had encountered five years before. The world was in chaos as the virus swept through city to city, town to town. No cure was in sight. Eventually, those who had survived formed groups to protect themselves from those who were infected.
Three years since Lucifer rose. Dean became the leader of the Resistance of those who wished to stop the Light-Bringer and his horde of demons. Castiel still stood by Dean's side, especially after the Detroit incident (Sam said yes to become Lucifer's vessel). Not long after that incident, Castiel began to lose his powers as his brothers and sisters left earth. He had no idea where they went and each day they were gone; his powers diminished, eventually he became all but mortal. Castiel had tried to leave Dean, for he was useless now that he was mortal but the Hunter, now General of the Resistance, refused to let him leave him. So, Castiel stayed with his charge/friend, helping out when he can. The Resistance did not know of his true nature and Dean wanted it that way.
Five years since Lucifer rose, Dean had become a changed man, a harder man. After the Detroit incident, he had become colder toward those he did not trust, which was everyone except Castiel, Ellen, Jo, Rufus, Bobby and Chuck. Those outside of this close-knit circle suffered Dean's wrath when missions did not go according to plan. Dean's close-knit friends dwindled down to two: Castiel and Chuck. The others had been killed either by the Croats (those infected with the Croatoan virus) demons, or just plain old age, which was rare.
In order to keep alive, Dean reverted back to his torturing ways, as he gathered information from demons he and Cas had captured. Castiel did not approve of Dean torturing again but he conceded Dean's point when he said they needed to survive at all costs. "I have to Cas. Will you stay by my side when I do this?" How can he leave Dean after a plea like that, besides the thought never crossed his mind. Soon after Dean's plea, Castiel found himself enjoying the torture. He knew it was wrong, but he was tired of it all. He could no longer feel the wrongness of torturing God's creation.
God was dead.
God was not real.
Dean and Raphael were right...
Castiel and Dean no longer discerned the difference between demon, Croat, or human. They killed and tortured them all. Survival of the fittest.
Five years and eight months after Lucifer's rising, Dean came up with a plan to kill Lucifer once and for all, but it was a suicidal plan. There was no guarantee they would survive, but they had to stop the Devil. Dean's plan was to find the Colt (a gun that can kill any creature, including the Light-Bringer himself) and use it on Lucifer. They found the gun, now all they needed was to find Lucifer. With a stroke of luck, they found a demon in Lucifer's "entourage" and he spilled everything to Dean...how can he not? Dean was an accomplished torturer. They made their plan and come midnight, they would kill the Devil. Dean paid Castiel a late night visit, confessing his every fear, lie, and hope to the fallen, now mortal angel.
"I don't know if I can pull this off Cas." The Hunter/General confessed. "I don't know if I could do it."
"You can do it Dean. I have faith in you. I know I haven't said that in a long time, but I thought I didn't need to. Not anymore. You earned it." Granted, he was still a little high, but the Hunter understood he meant it.
"Thanks Cas." The General said gratefully, letting the hard mask he wore these past five years fall, showing how his friend had aged through this whole ordeal. The smile the mortal angel gave Dean made him return the smile in kind. A sincere, real smile graced the Hunter's face for the first time in ten years. They passed the remaining time in companionable silence.
Dean had failed.
The Devil is still alive.
Dean had failed...
Castiel had somehow managed to survive the trap the demons laid out for the Resistance. Castiel knew by Dean's face, he was using them as decoys. When he caught the Hunter's eye, he saw the truth within those hazel orbs; Dean was using them, using him. But he also saw the regret in them and Castiel forgave him. He nodded his forgiveness and consent to the Hunter before he charged in with the motley crew of "Hunters".
Inside the warehouse, it was all a blur for the angel. He vaguely remembered shooting, getting cut, getting choked, being thrown into a wall and then finally blacking out from some odd injury or other. He did not know why they left him alive and he cursed them every waking moment. The Resistance was no more. The demons and Croats found them. Everyone was dead. He was all alone. Lucifer had won. Earth was his.
A millennia had passed since Lucifer's win over the earth. The land was barren and dead. The sky bled. Demons walked the earth with their "Father" at the helm. Either by Lucifer's twisted way of apology or his brothers and sisters' way of punishing him for letting the apocalypse happen, he was still alive. Castiel was the only living mortal being left. For years he wandered the land, looking for something thing to keep him going. He never found it. He was alone. How he hated being alone. For the first time since his creation, he was on his own. Before, he had his brothers and sisters then, after them, he had Dean. Now he has nothing. The quiet was killing him. The memories of the past kept him going for awhile. Now, he found himself waiting. He wandered the desolated land waiting until the time Lucifer gave him what he wanted: to be with Dean wherever he is and to be at peace.
3,000 years after the Fallen and his "people" left earth, he sees the Devil walk towards him with a gentle smile; Castiel allows a smirk to grace his face.