Mask of an oni woman used in Noh plays. The mask is used in the staging of the dramatic scene in which a beautiful woman, represented by the smiling "koomote" mask, changes her appearance to that of an oni as she struggles with her inner conflicts.
The word "Han'nya" originally meant "deep insight (prajna)" in Sanskrit, but this has nothing to do with the name of the mask itself, and there are several theories as to why the demoness mask came to be called Han'nya.
One theory is that the name was given because of the excellent work of a monk named Han'nya-bou [Honofiric used for monks.] who made oni masks at the end of the 15th century. There is also a theory that says that Han'nya-bou was the first to make Han'nya masks, but this is considered false because the masks themselves existed before that time.
Another theory states that the Han'nya masks were used in "Aoi no Ue" in order to drive away wraiths through the Heart Sutra (Hannya Shingyo), which is said to have miraculous powers to ward off specters. One more theory claims that the name was originally "half-snake (hanja)" but was changed to "Han'nya".
The Han'nyas used in Noh are: "Namanari" used in "Kanawa", "Chuunari" used in "Aoi no Ue", and "Hon'nari" used in "Doujouji". [Every name ends with the Kanji for 'become'. Namanari means 'unfinished', Chuunari likely means either 'half-changed' or 'still-changing', and Hon'nari probably means 'truly-changed', so each mask could be taken as a representation of the stages of becoming an oni.]
In "Doujouji", a woman accuses a man of infidelity and turns herself into a serpent to burn to death the man, who has escaped into a bell, hence the mask is said to represent a snake.
And in "Aoi no Ue", "Chuunari" halfway became a snake, thus the mask was called Hanja (half-snake), which contributes to this last theory.
