The Oni is a giant demon with horns and bared fangs, commonly drawn with red or blue skin and a tiger-skin loincloth. It appears both as a hell-warden punishing sinners and as a monster menacing the human world.
What makes it especially frightening is the belief that a wicked person can become an Oni while still alive. The idea that extreme emotions like hatred and jealousy transform a human into a demon carries the horror of a monster that grows not from outside but from within.