After the loss of his parents, Ciel Phantomhive initiates a contract with the demon, Sebastian Michaelis. According to the terms, the demon will help the young Earl exact his revenge on those responsible in exchange for his soul. This straight-forward premise carries the initial 24 episodes of "Black Butler" right up to the season's closing OVA.
Season two introduces another master/demon butler duo. These episodes track the arc of Alios, a sadistic youth seeking revenge for the destruction of his village, and Claude, the demon he contracts. During the unfurling of their arc, the plot muddles about unraveling the how and why Sebastian and Ciel are even appearing in the story, considering the conclusion of season one.
I have to admit, after the opening two episodes of season two I actively researched spoilers because otherwise I would have been so hung up on the seeming incongruity that I would not have been able to enjoy the season. Needless to say, in the end, I did not find that the second season flowed as coherently as the first.
There are also only twelve true episodes addressing the central plot in season two, which, it ends up, was sufficient enough to round out the Alios/Claude arc while bring the ongoing story of Ciel and Sebastian to some kind of logical close as well. After the twelve episodes, six OVAs round out the season.
Overall when considering both seasons, I found the two central characters of this anime compelling, if not simplistic in their single-minded motivations. Ciel, the twelve-year-old Earl, is basically a revenge seeker executing his duties as the Queen's "watch dog" to pass time until his vengeance is complete. His soul's true motivation is such a driving force in his life that at one point he questions if he can exist without it and the requisite hatred his need for revenge cultivates.
Sebastian, ever dutiful in his role as butler, is motivated by his desire to consume Ciel's soul. Yet, there are moments, especially in season two, when outside forces claim that there clearly is more than demonic hunger at work. Even in season one there were moments when it did seem respect, and perhaps even fondness, for Ciel had a hand in Sebastian's unwavering loyalty to his young master.
In addition to these two central characters there is a support cast that at times adds levity to the anime, lightening the overall dark theme of the series.
Black Butler began as a manga published back 2006. The first season anime adaptation followed two years later in 2008 with a second season in 2010. Funimation later acquired the rights and the anime debuted in North America in 2011.  As of 2014, a third season and live-action film are added to the canon. 
In conclusion, I would recommend this anime to anyone who enjoys darker themes that are conveyed with a modicum of humor. In other words, to paraphrase Sebastian, "Black Butler" is "one hell of a[n anime]!"
If you end up enjoying "Black Butler," I would also suggest "Soul Eater." The English dub features a few of the same voice talent as well.
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