Riddle Story of Devil

At the private girls' boarding school, Myōjō Academy, thirteen girls are transferred into the academy's "10th Year Black Class". Of these thirteen students, twelve are assassins from various backgrounds who are all tasked with assassinating the remaining student, a girl named Haru Ichinose. If they succeed, they will be granted any wish they desire, but if they fail, they are expelled from the class. However, one of the assassins, Tokaku Azuma, has sworn to protect Haru from the other assassins.

"The world is filled with forgiveness."

Akuma no Riddle (悪魔のリドル, lit. Devil's Riddle), also known as Riddle Story of Devil, is a Japanese manga series written by Yun Kōga and illustrated by Sunao Minakata, licensed in English by Seven Seas Entertainment. An anime adaptation by Diomedéa aired in Japan between April and June 2014.

Tropes used in Riddle Story of Devil include:

Assassin Outclassin'
The Clan: There are two assassin clans which have existed since ancient times: the Kuzunoha Clan of the West and the Azuma Clan of the East. Tokaku is the heiress of the Azuma Clan, and Nio is the heiress to the Kuzunoha, who adopted her as a little girl.
Deadly Graduation: As revealed at the end of the series, the Black Class is an elaborate Rite of Passage for Haru's clan, to determine if she had what it took to be a primer. The test itself did not necessarily involve killing her best friend, but she did have to survive to graduation, and that meant she couldn't just let Tokaku kill her either.
Dwindling Party: The nature of the competition to assassinate Haru means that this was inevitable.
Elaborate University High: Myōjō Academy.
Idiosyncratic Episode Naming: The episode titles are riddles.
Knife Fight: Between Tokaku and Isuke.
Punny Name: The Japanese title is a pun on the Devil's Trill Sonata by Giuseppe Tartini.