Hannya is a mask that is used in Japanese noh theater, representing a terrible grin of a jealous woman, demon or snake, in her upright position. Another explanation is that the perfect wisdom of the sutras and their variations was considered especially effective against female demons. An alternative explanation is that the craftsman needed a lot of skill to create this mask.
Just like with these weapons, it takes a lot of skill to make it deadly...

-Did you see it? Something flashed in the darkness...