"Devilish Impulses" by Kitty Gets Loose
I decided to make this list with this particular story in mind because it needed to be on a rec list. Dozens of rave reviews (several of them mine) just aren't enough for a story of this calibre.
Elaborating that last statement further, Kuroshitsuji has, I think, somewhat of a bad reputation as far as fanfiction goes insomuch that it has become a breeding ground for less-than-stellar fics. That is, many authors seem to approach the series as if it canonically portrayed or hinted at a secret romance between Ciel and Sebastian when, clearly, their actual relationship is anything but.
Kitty does not make that mistake. She starts her story and the characters off as they were in canon and gradually, believably, develops the story and the relationships further.
"Devilish Impulses" begins where the first season of the anime ends: Sebastian poised and ready to devour Ciel's soul. However, Sebastian, upon seeing what can only be described as fearless eagerness from Ciel, decides not to eat him (You wouldn't want to eat a cow that was acting strangely just before you shot it, would you?). Sebastian also realizes that he was rather amused by Ciel during the time they spent together, and doesn't want to lose his charming pet just yet. Ciel is angry, but eventually accepts a new deal: Sebastian will continue to assist him for the next five years contract-free.
Over the course of the story, Ciel comes to terms with still being alive and decides that if he is going to continue living --at least for another five years-- he is going to change the way he lives. He seeks to separate himself from the title of The Queen's Guard Dog and the London underworld. He will still fight for justice, but less so in the shadows.
Sebastian, over the course of the story, realizes that his view of Ciel has gradually been changing: from amusing pet, to a show horse to be groomed and compete for prestige with, to something else. And it is that something else that ends up threatening to tear them apart.
Aside from the wonderful character- and plot-development, the bearable and even enjoyable original characters and intelligent humor, this story his a history junkie's dream. Kitty really does her research for this one. The dialogue is correct for the period, as is the technology and clothing. Her attention to detail is absolutely mind-blowing. Victorian history buffs, eat your hearts out.
Most importantly (or at least what I consider so), this is not a typical shota fic. It is everything a shota fic is not: darkly humorous (not creepy), intriguing, and seductive. Ciel is not some pliant, teary-eyed doll, and Sebastian is neither a rapist or magically The Best Boyfriend Ever!!1!. Yes, Ciel may be a child, but he rarely thinks like one and even rarer likes being treated like one. He is stubborn, cynical, and still in charge.
Fans of Black Butler, whether they particularly like Sebastian/Ciel or not (I don't, most of the time) should read this story. It is at times, honestly, better than the manga, and is without a doubt, the best fic in the entire fandom. Not kidding. And it's still updated every Wednesday. Just read it, read it and let yourself be grabbed and pulled along with it like I have been the past few months. Also, check out the reviews: they contain a lot of good insight. {Sebastian/Ciel, drama/romance, NC-17, WIP}