To be honest, I'm not entirely sure what this is. Hope you like it.
It never truly escaped Ciel Phantomhive that Sebastian was, in a way, his own creation. Without a master, Ciel theorized, Sebastian was likely nothing more than a shadow of chaos and hunger of the worst human kind. Sebastian looked the way Ciel wanted him to, acted the way he wanted him to, cared for him the way he wanted him to, and bantered in a way that matched Ciel's own wit.
Sebastian was a pleasant representation of Ciel's death. He would make Ciel comfortable, comply with Ciel's death wish, ease the transition from existence to not. Take his last reason to exist away from him.
As Ciel grew older, he saw more and more of his father's face inside of his own. Although he didn't quite have the sly attractiveness that his father had (and it was no coincidence that Sebastian possessed the same quality), he could at least say the bone structure was similar. It became more reasonable for him to be the Earl Phantomhive, as the face of the family name had reached the age of twenty, and was no longer thirteen. A more menacing guard dog, it would seem. Business deals, at least, were slightly easier.
Despite his increasing age, Ciel could never shake the mindset of the battered, malnourished, corrupted boy that had desperately summoned his own demise, and welcomed it into his cage. At times, this was beneficial in regards of saving face and running Funtom. He was calculating and manipulative, and could talk businessmen out of their money like nobody's business. Other times he would find himself seized with a rapid heartbeat and burning lungs at the slightest reminder of his time in hell.
This would most often occur at night, after waking from a nightmare. And after a few years of dealing with this, Ciel lost just enough of his pride to allow a demon into his bed to comfort him. He found himself addicted to the cool touch of Sebastian's soothing fingers. Ciel would lie there, his ear to his butler's chest, and contemplate how interesting it was that the demon, in this form at least, had a heartbeat. In tandem with the soothing strokes Sebastian would administer to Ciel's back, the notion was often enough to coax Ciel into a more peaceful sleep.
Following Ciel's marriage to Lizzy, such an action would become more complicated. In his own way, Ciel did love her. And as a result, he didn't want her to have to deal with who he really was. He knew Lizzy was in love with the Ciel she had known in early childhood. Not with the broken boy he was now. She wouldn't quite understand his episodes, and she would have little to no way to help him. Really, for as long as he had left, he wanted her to have the happiest marriage to him that she could, as he felt it was the best that he could do.
So when Ciel woke from a nightmare less than forty eight hours after their wedding, trying to keep his heavy breathing to a minimum, he slid out of the bed, careful not to disturb her, and slipped down to Sebastian's chambers, hoping that Sebastian really was an extension of himself, and would be what he needed him to be right then.
The door opened before Ciel could knock, and there stood Sebastian, still in his uniform, slightly bent over. He said, "It's quite late my lord, what can I do for you?"
Despite wanting to say something more dignified, Ciel replied in a weak voice, "Can I stay with you?"
Sebastian glanced up, and saw the tears threatening to fall down Ciel's face. He scooped Ciel up, as easily as he would have if he were still a child, and carried him over to the small rickety bed that took up most of the room. "I apologize, my accommodations aren't quite as comfortable as you're used to," Sebastian said.
"I-it's okay. They'll do."
He laid Ciel down and settled in next to him, wrapping his arms around the Earl's waist, and stroking up and down his back. "Another nightmare, my lord?" Sebastian whispered. Ciel sighed in a sort of affirmation that Sebastian didn't really need. Ciel's breathing began to quiet as Sebastian chuckled softly to himself. All the masters before Ciel had been fleeting. None had lived quite so long controlling the demon. None had trusted him as much as Ciel did, and none trusted him as little as Ciel did. He wondered if he would encounter another contractor quite as unique as this one in another few millennia. He also wondered how long it would take for Ciel's more carnal desires to awaken, if they would at all. Twenty, married for three years, and yet to produce a single heir. And if the beautiful woman upstairs is not what he desires then, well, what's a devoted demon butler for?
Ciel slowly fell into a dreamless sleep to the rhythm of Sebastian's heartbeat. And when he blinked awake to birds chirping, he was startled to find himself in the embrace of his wife, in his bed. Realizing it was barely six o' clock, he settled back in and sighed to himself. Of course Sebastian would return him here where he could do more damage or none at all. No one is less trustworthy than a demon, and no one is more trustworthy than death.