The mask on the poster and in the film is a Hannya mask, used in certain Japanese theatre. While the name comes from the word for “wise” it’s not referring to the mask itself, but supposedly to the artist who made it. Focusing on the wisdom needed to perfect the design.
The mask is used to portray women who have become demons due to jealousy, but there’s so much more depth than just some troll with horns. Look at it…