The Kuzunoha of the West
The Kuzunoha Clan (葛葉一族; Kuzunoha-ichizoku) is one of two assassination clans that existed in the country since ancient times, the other being the Azuma. The Kuzunoha are the main branch who rule over the western territories of Japan while the Azuma is a branch family and they rule over the eastern territories of Japan. They supposedly supported powerful groups that dictated the course of history from the shadows. However, interactions between these two clans have almost died out. It is mentioned in the drama CD that there was some conflict between the two clans, but Nio dismissively says that was a long time ago.
The Kuzunoha specialize in shamanism. According to Nio, in practice, it is close to hypnotism and suggestions as members of this clan can take mental control of their target, steal their correct perceptions and chase them to death, or even make them kill others.
Unlike the Azuma, who retain an unbroken bloodline of succession, the Kuzunoha clan sustained itself by adoption of those who possessed desirable abilities or acquired them, much like succession schemes in swordsmanship schools.
Not much is revealed about the Kuzunoha as a clan in structure or operation. Yuri said "Kuzunoha-san" gave Nio to her as a servant. Nio had no name until Yuri named her, so it is implied that people in the clan might be nameless to a point.