Black Butler has to be one of my most favorite animes of all time, it was one of the first mangas I’ve ever read as well.
Black Butler is about a boy named Ciel Phantomhive, in the late 1800s of England. Ciel is wealthy and lives in a large manor, his parents are close to the queen and he is also.
On Ciel’s birthday, the manor is lit on fire and his parents die. Ciel is then kidnapped and tortured by an unknown organization. While Ciel is being tortured, he calls something in his mind, making a demon appear. He makes a deal with that demon. To get revenge on all of those who betrayed him and his parents, the demon will get his soul (Which will kill Ciel.)
The demon is named Sebastian and then is made a butler to the Phantomhive house. The two of them work to get revenge on enemies, figure out mysteries, work for the queen to solve crime and much more.