"Supposedly Nobunaga had his generals drink from the cups and then forced his wife who came from the Asai family to drink from the skull of her brother. While the taking of heads is nothing unusual, after they are presented to one's commander for reward for service in battle, they are usually given proper services and buried. The decoration of skulls in this manner does have some precedent in Chinese history and that may be where Nobunaga got the initial idea from. Still, the display of skulls as objects of art to be admired was probably a shock to many of those present. Probably that is precisely the effect that Nobunaga was hoping for; to make an impression on those present that he was not going to be bound by convention and the rules as previously understood, did not apply to him. He would go to any length to achieve unification and the creation of a unified and peaceful country justified whatever means he chose to employ. Doing the unexpected, the shocking, or outrageous was one of Nobunaga's favored strategies as it kept even his closest vassals off balance and unsure as to what he might do or how he might react. While on one hand, such acts did create an atmosphere of fear even among his top vassals; they also kept everyone on their toes and on their best behavior."
The Rules of Engagement did not apply to Nobunaga and he did scare the Bee-Jesus of his enemies and his vassals. One must remember that the sword was mightier than then pen in Sengoku Japan. For Nobunaga: It was my way or the highway!
Nobunaga no tame!
Tenka no tame!