It's hard to be a hallucination. Lucifer could see now why the Winchesters had been so reluctant to do their part in the apocalypse. To exist only in the back of someone's mind, pushed away and suppressed, trying to desperately cling to the mere existence was straining. And hope that maybe, just maybe, one day you could break free and take control, to force your captor down and reverse the game. It had been boring and long, for the most part. Of course he had had the privilege to see and know things that only Sam had seen. It was a small plus side to the whole ordeal.
When Castiel had come to steal Sam away from him, Lucifer had grabbed tight onto Sam, any part of Sam, and refused to let go. He had succeeded, in a way. He had been able to keep Sam's soul, if not all of him. After all, the soul was what was most of Sam. The part that he could have fun with, play with endlessly as years went by. But he hadn't been allowed even that for a very long time as soon Death had descended into the Pit and taken Sam's soul with him. Lucifer hadn't wanted that to happen. Sam had been the only thing in there to keep him entertained as Michael had been a douche and kept his distance to Lucifer, his vessel neatly tucked under his arm. So Lucifer had fought back, wrapped around the mutilated thing that was Sam's soul and hold it close with all of his might. It hadn't been enough; Death had taken Sam and Lucifer had felt almost hollow, like a part of him was missing.
It turned out that had been literal. Lucifer wasn't new to the fact that every time an angel leaves a vessel, part of that angel's grace sticks with the body. But what he didn't know was that when an angel held onto a human soul as desperately as he had, he would not only leave his claim onto that soul, but also willingly give more of his grace to it. Whether the soul wanted it or not.
So even when Lucifer was left alone with his mood-swingy older brother, part of him hid itself inside the deepest cuts of Sam's soul, holding onto the familiar warmth. Death, of course, saw exactly what had happened, had even anticipated it. And so he put up the wall inside Sam's mind, forcefully separating those little pieces of Lucifer from both his soul and body, and never told the truth about it to anyone.
And until Castiel broke that wall between Lucifer infested parts and the rest of Sam, no one was the wiser. But when that first crack hit the wall, when Sam fell limply on the ground, Lucifer was struck with a familiar feeling of Sam. Tiny little cracks had appeared in front of him, inviting him in, allowing him to seep just a bit through to the other side. It was only enough for him to see Sam, feel his body and soul. But it was enough.
N/A: This is just a Prelude, the actual First Chapter will be the second and is where the story actually begins. I'm hoping to publish new chapters as often as possible and not have more than two weeks tops between chapters. Let's hope I can keep it up.
This story will be mostly unbetaed. I'll proof read every chapter, but english not being my native language, I apologize if there's mistakes. Please feel free to point them out to me if you find them. :)