There was once a hunter, who, in effort to defeat a Knight of Hell, took on the Mark of Cain and became a Knight of Hell himself. He was, eventually, saved by his family – his brother and their angel. But, while he was cured of being a demon, he still bore the Mark, and it still tormented him. His brother was desperate for a way to take that burden away.
Eventually, he found a way, though that way promised the potential of even worse than the Mark being released on the world. This was his beloved older brother, and the risk was worth it to him. It would always be worth any cost. He enlisted the help of the angel, who, though he had his doubts, did as he always did, and worked to save the Winchesters...even from themselves.
Their plan, for better or worse, worked. The hunter was freed of the Mark of Cain. But, it came at a cost. Along the way, their little sister, adopted to their little family just as the angel had been, lost her life to see the hunter freed of the Mark. The angel, well, he nearly lost his life as well, and suffered a terrible curse cast on him by the witch they'd made a deal with to work the powerful spell to eliminate the Mark. Even Death, in the end, lost his life to the powerful bond between the brothers, those infamous Winchesters.
But, the worst cost came in what was released when the Mark was destroyed.
The Darkness swept across the land. But, the Darkness was like nothing anyone could have imagined. It...was a she. And she was born into the world in the body of a tiny baby, her chosen vessel taken at the moment of the child's birth. But, even as an infant, where she went, Creation became warped. At first, it was an infection that seemed to turn people into something very like zombies, an infection that claimed the life of the tiny baby's father, and nearly claimed the life of Sam Winchester, as well.
Sam, during that brush with death, reached out in desperation. He prayed to God. He reached out, and he felt he'd gotten an answer. And in the following weeks, he continued to feel he was answered by God, guided to a solution to the Darkness he had helped to unleash on the world. And, though it terrified him, he bowed to what he felt God was showing him. Sam, with the help of the King of Hell and his mother, the very witch that had worked the spell to destroy the Mark that had kept the Darkness locked away for eons untold, went to seek out Lucifer in his Cage.
Lucifer was everything Sam had remembered. Just as devious, just as seductive, and just as manipulative. Sam, it turned out, had been fooled into believing God was communicating with him, trying to help him in his search for a way to right the wrong he'd brought into the world while trying to save his brother. In the end, it had only ever been Lucifer speaking to him, leading him on.
But, Sam resisted Lucifer's demands, his seductive reasoning. Once, Sam had said yes to Lucifer, and he was determined to never do so again. He refused to bring another evil into the world, even if it meant the defeat of the Darkness. So, he said no. He said no, and waited for the torture to begin. He said no, and waited for his faith in his family to bear out. Sam's faith was well-founded. Dean came for his little brother, just as he always had. Dean came to rescue Sammy from the Devil, and he brought their angel with him.
But, what neither brother knew, was that their angel, the fallen angel Castiel, was broken in ways neither Winchester could have conceived of.
Castiel was determined to save Sam, as Dean was. But, Castiel was also determined to save his Father's Creation from the wrath of the Darkness. What neither brother could have ever imagined was that their friend, their brother, the fallen angel Castiel, was willing to give himself, everything he had, everything he was, to set right the wrong that he had so willingly participated in.
They only had to hold out against Lucifer until Rowena, that same witch that had helped unleash the Darkness, worked her spell to lock Lucifer away in his Cage once more. They didn't have to defeat him, for they knew they were ill-equipped to defeat the Devil himself. They only had to hold out against his violence and his threats and his manipulation until he was swept back into the Cage.
However, Castiel felt that it wasn't enough. In the time before, it had taken his Father and his first angels – the archangels Michael, Lucifer, Raphael, and Gabriel – to lock the Darkness away from all of Creation. Michael had inadvertently been locked away in the Cage with Lucifer to end the Apocalypse, and was now insane and unreachable. Raphael, well, he had been destroyed by Castiel himself. Gabriel, he was thought killed by Lucifer because he'd stood in the way of his more powerful brother during the Apocalypse.
Lucifer, Castiel understood, was their last hope. And, like Dean, he didn't want to see Sam put through being ridden by Lucifer out of Hell. Sam had given enough. Sam still had usefulness and value in the story. But Castiel? Cas was broken, a used up broken tool. He was expendable. He didn't have value in the story anymore. But, maybe, he could be of one final use. So, he asked Lucifer desperately, “can you really beat her?”
Lucifer, grasping at the chance offered him, seeing immediately in his younger brother's broken expression what he meant to do, said with determination, “I can.”
“Then, yes,” said Castiel, meeting his brother's eyes in what he felt would be his last act.
Lucifer took possession of his brother's vessel, a vessel that was actually strong enough to hold his immense power, having been rebuilt more than once by their Father Himself to house his beloved child's grace and form. After all, in so many ways, Castiel and Lucifer were reflections of each other. Both had rebelled for love, both had set themselves on courses of damnation with the bad choices they'd made for what they felt were the right reasons. Castiel was the light reflection of Lucifer's darkness. It made Castiel's vessel perfect for Lucifer's use.
And Lucifer made perfect use of it. For some time afterward, he pretended to be Castiel. He fooled the Winchester brothers, and he sought a way to defeat his Father's Sister, Amara, the Darkness. He found that way, eventually, in the most unexpected of places; he found it within himself. Castiel, locked away deep within the vessel, turned out to be far more useful than Lucifer could have imagined. This broken little angel, it turned out he hadn't been quite as ready to give up as Lucifer had thought.
Once the little angel had gotten over raging against him, raging against ignoring or fooling all those Cas cared about, Castiel started to speak to him. And Castiel spoke to him often, even going to far as to try to offer suggestions where in the bunker's immense and primarily uncatalogued wealth of lore and texts to look for help. After all, he'd given everything to see the Darkness defeated, he wasn't going to stop with just giving up his vessel, with just letting Lucifer ride him.
He eventually convinced Lucifer that humans had value, that humans could be – in terms Lucifer could deal with – useful. And, though it was slow, Lucifer did begin to see that humans could be useful...and could be used. Castiel convinced him to reveal himself to the Winchesters, and, with much mistrust on all sides, they eventually worked together to find a way to defeat the Darkness.
The last piece to the puzzle of defeating their mutual enemy came unexpectedly one day, in the appearance of someone none of them could have ever expected. Gabriel. He had managed to use all his tricks to escape Lucifer's deathblow, and had been in hiding ever since. But, eventually, even he had to realize that the Darkness rampaging over the Earth was something that had to be dealt with. Much like Castiel, his encounters with the Winchesters had changed him. He couldn't just stand by and watch the world burn anymore. He had to take a stand.
It took the combined might of these two remaining archangels – where originally God had intended that there be four existing – plus the sheer determination of the Winchester brothers, and the addition of the broken little angel Castiel's grace and love and good intentions to bind the Darkness away. Even then, it wasn't complete. She still stirred in her locked away nothing place, waiting, sometimes even managing to reach out and bring darkness and chaos to the world, in limited ways.
But, in the end, the Darkness was locked away by these five heroes.
They never spoke of it again, not one of them. They never told the tale of how Castiel came to be free of Lucifer's possession. They never spoke of what happened to Lucifer. They never spoke of Gabriel simply vanishing, nor did they seek to find him – leaving him in peace until he chose to return was the least they could do for him, for his help. Occasionally, some curious hunter the Winchesters ran across who heard the story might ask about these things. They would, always, just shake their heads with lips pressed tight, refusing to answer. Instead, they would just reply, <i>“Everything changed that day.”</i>