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."

Tropes used in Riddle Story of Devil include:

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 entire 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 didn't explicitly 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.