18+ Kage (OVA)

It is the peaceful Edo period in Japan, and ninjas are hardly needed anymore. Karyu, a particularly skilled ninja, is forced to do work as a hired killer. She is asked to kill a merchant, but when she gets there, she finds that his servant is actually a ninja, who is just as skilled as Karyu. Her name is Uzume. Karyu and Uzume then compete against eachother in a bloody battle for superiority.

(Source: ANN)