Onibaba

"People are both the devil and god--and are truly mysterious"

Onibaba (鬼婆) AKA Demon Hag is a Japanese Psychological Horror Samurai film from 1964, written and directed by Kaneto Shindo. It is based on an old Buddhist Fable, but the film is notable for its uncompromising modern vision, including frank depictions of sexuality and ruthless female power, fast cuts, slow motion, unusual angles, and a frantic soundtrack mixing natural sounds and traditional drumming.

It was filmed in a field of high susuki grass, meaning that neither the characters or the camera were able to see more than a few feet, which helped to create a claustrophobic, entrapping effect. The action takes place during one of the many bloody wars of Japan's feudal period, but centers around a small group of poor fishing villagers who must eke out an existence on the outskirts of the brutal war, highlighting the social conflict between the ruling samurai and the poor farmers on whom they built their empires.

The iconic mask inspired the design of the demon in The Exorcist. Also, the film is named after creatures in Japanese folklore, which you can find here.

Tropes used in Onibaba include:

Becoming the Mask: Literally, when Kichi's mother's attempt to scare her daughter backfires.
Black Widow: Kichi's mother.
Clingy Costume
Cool Mask: The mysterious samurai Kichi's mother leads through the susaki grass.
Crapsack World: The peasants are trying to survive while surrounded by armies who destroy their fields, take their crops, steal their daughters, conscript their sons, and where wandering deserters haunt the roads.
Dangerous Deserter: Groups of them pass through regularly, and they're often hungry. Hachi, himself is one of them, despite being somewhat more sympathetic than the norm, he is still a desperate, untrustworthy criminal.
Demonic Possession: Or is it?
Evil Mask
Lovable Rogue: Hachi (at least, Kichi seems to think so)
Mrs. Robinson: Kichi's mother makes a pass at Hachi.
Pit Trap: Deviously hidden in the high grass.
Ronin