To bring in more text to the movie and people seeing "the mask". The mother removes this mask off a Samurai that wasn't fully dead that they assumed they had killed. Before she ripped it off his face, he warned her that the mask is cursed when it rains.
She is eventually consumed by her own curse as well as the curse of this mask.
It's a really good movie. Not a whole lot of action really. There's no fighting for the most part aside from two women squabbling.