忍びの卍 (1968)

When the Shogun Tokugawa III is in bed with a girl, she turns into a monster. The court believes she is Ninja Magic, used to frighten the lord off women so that there will be no heir. Tôma, a famous swordsman, is called in and exposes the Ninja, even discovering their leader, a beautiful young girl named Kagiroi. Guessing that she is the daughter of some ousted daimyo anxious to get back into favor, he is about to unmask her when he is taken prisoner...