I actually found this anime thanks to Amazon recommending it too me, as soon as I saw the front cover I instantly recognized some characters I saw some people cosplay as at a comic con, so I decided to purchase it.
It's basically about a boy who traded his soul with a demon who serves him as the most loyal and amazing talented butler anyone could dream of.
As he's a demon you can expect a dark and twisted side about him whilst being rather humorous, he gets rather upset/disappointed when he can't get something perfect or dinner on time.
There are some other funny characters that live in the mansion who look up to Sebastian, but they're extremely clumsy, the maid you'll either love or hate as she has a very stereotypical cockney accent which I personally find hilarious.
Overall, I really enjoyed it and would recommend it, for those that like dark anime's like Death Note
, then you'll probably like this one.