Sebastian often wondered when exactly his demon nature began to wane. Maybe after realizing he didn't have the heart to eat Ciel's soul… Which meant he felt at least something close to love for his master. Then there was the day he cried for the first time, as he was unmistakably elated. Ciel's first child was born. The little thing looked just like his master, which meant looking after two Bocchans. After putting up with Ciel's childish behavior for so long, it didn't matter anyway.
The thing is, he was becoming more and more human as the days went by. Tears wouldn't stop as well when the other children were born, then the grandchildren. All the while he wanted his master all to himself. Yes, powerful demon Sebastian was feeling jealous! There was also the wife, that damned Elizabeth. Not even one thousand kittens were able to drive off these feelings of possessiveness.
One day, the demon wondered if it was a starving side effect. So he won't lie: one night he snuck out of Phantomhive's mansion and ate a couple souls. But alas, the emotions wouldn't go away! What a pain...
But everything was going as usual in the Phantomhive household, as the Earl was successfully managing his toy company while also keeping his queen's guard dog function in check. Ciel gradually started to smile more often and also turned less calculating, learning to enjoy other people's company more and more. It made him even more precious to the butler, who, as the days went by, became one of the few to know his truly devilish nature.
Well… everything was fine until that fateful day, that is.
Ciel, aged 60, caught a vicious pneumonia. His lungs were never in perfect shape to begin with due to asthma, so any other breathing problem would certainly be fatal.
In his deathbed, the once powerful Earl couldn't let his last secret die with him. "Sebastian…!", he called rather forcefully after making sure there wouldn't be anyone but them in the room.
"Yes, master.", the butler smiled softly, yet sadly.
Ciel murmured between coughs, "I… figure… I won't last long. Let me see the young you one last time."
Sebastian seemed relieved. The demon's human form was quite aged up, as he pretended to be as least 90. To be frank, pretending to be a weak old man was a hell of an annoyance.
"Yes, that's the you I first fell in love with…", no matter how embarrassing the situation, Ciel's smile couldn't be any more smug.
The butler laughed nervously: was it real? Then he replied playfully, "My, my. Do you actually love me? First time I've heard of it. You mean you enjoy ordering me, no?"
"No, I'm deadly serious." the Earl chuckled between coughs. "the truth is, Lizzie isn't the one who makes my heart beat faster".
Well...what a predicament. So they were a demon in love with a human and a human in love with a demon. As confusing as it was, at least they loved each other.
Seeing his beloved so battered on the bed, Sebastian stated the only thing he could think of to ensure them a 'happy' ending: "Will you make another contract with me?"
The Earl seemed rather amused. He wondered what his butler was planning."Only if it means you will love me back, you damn demon."
The butler chuckled. They were on the same wavelength, actually they have always been so. "Well, well… will you be my groom?"
"What…?!", Ciel was certainly surprised. After loving the demon for so long, he didn't expect it to be this easy.
"First of all, I must explain how it works. We shall have eternal youth under the age we see most fitting. No natural death, no disturbances. Lying to each other is strictly forbidden. The contract can never be invalid, even if one of us breaks it. We decide any sort of punishment by ourselves".
The Earl agreed while making the utmost dreamy face.
"And for all of this to happen… I'll have to turn you into a demon".
Ciel surprisingly wasn't taken aback by this,"After all I've done as the queen's watchdog, I can't say I'm really human, can I?"
Being so sure about everything… Sebastian treasured that side of him.
"Will you gladly become bound to me forever, as even eternity can't do us part? Will we switch from butler and master to equal demon souls illogically bound by love?"
Ciel agreed, stating frankly,"No power play for us anymore."
And as soon as Ciel became a demon, they sealed their vows with countless kisses.
Demon yet human, two of a kind. They say even now this power couple turns the underworld upside down, unpredictable being their middle name.
The demon...and his hell of a groom.