A/N: I was originally going to make this story more graphic, but it seemed to have less of an emotional impact that way, so I stuck to pure emotions.
It wasn't easy going to sleep with Jack in the bunker. The first few nights Sam only got around an hour of sleep, maybe two. He would lie awake, tossing and turning, staring at his door. He was frightened of him, and he felt guilty about that. Jack was just a kid, not even a week old at this point. And Sam was frightened of him. It wasn't just his powers, it wasn't just what he could do. It was the fact that he was Lucifer's son. Anything connected to Lucifer put a bad taste in Sam's mouth, and since he himself was connected to Lucifer that taste was often there.
Sometimes he lay awake, staring at the ceiling, or clutching his pillow with his head resting on it, remembering the horror that had swept through him, nearly claiming his voice, his movements, when Jack had called him father. Sam's first thought had been that he'd been mistaken for Lucifer, and it made him feel tainted. Though he now knew he'd been mistaken for Castiel he still couldn't wipe that dark memory away.
And then being in a cell with him had been difficult. Sam didn't fare well in enclosed spaces now, especially those that trapped him with a being that was much more powerful than him.
Sam felt stupid for having all these fears. He didn't know what Jack could do, or what he even knew how to do. It seemed like there wasn't much he knew of when it came to his powers. But they were still there. He'd seen them, he'd been attacked by them, attacked by Jack. But he knew it'd just been because Jack had been afraid. But if Jack was fearful enough Sam had a feeling his powers could level a city. He just didn't know it yet.
But there was more to it than that. He looked at him and he saw Lucifer. He didn't really look like him, but he felt like him. Beneath that naïvety and innocence and the good he hoped was in him, he saw something else. He hoped it was weaker than all the rest, was sure it was, but it was still there. There was a darkness in Jack that Sam couldn't deny. Sometimes he thought he only saw it because of what he'd gone through, and he felt bad about that. But he couldn't fight the way his stomach turned whenever Jack entered a room he was in, couldn't fight the way his muscles instantly tensed and he flinched, especially if he came closer without warning.
So Sam tried to make himself look smaller so that Jack wouldn't perceive him as a threat. The last thing Sam needed was to have another powerful being hurt him.
Even though he was trying his best to be there for Jack, to take care of him, the nightmares still came once he finally managed to get more than an hour of sleep at once.
The nightmares started out as they always did. He was in the Cage. With Lucifer. Or he was in the bunker, Lucifer in his room like he had been back when Amara had been trying to destroy the universe. But then he'd turn into Jack. And Jack would do the same things to him that Lucifer had done. He would beat him, slice into him, tear him apart, rip his skin from his bones. Touch him.
Sam woke up in a cold sweat from each nightmare, running a hand through his hair to get it out of his face as he sat up. And then he stayed there in the dark, throat aching as he tried to not cry. Sometimes he woke up and his face was already wet with tears, his pillowcase soaked. Other times he sat there feeling nothing but crushing darkness.
But then he'd see Jack the next day, and the kid would smile at him, and some part of Sam felt alright again.
Jack wouldn't hurt him. Sam knew that. But his scarred soul didn't. So the nightmares kept on.