Oni are ancient Japanese monsters.
Oni are one the greatest icons of Japanese folklore. They are large and scary, standing taller than the tallest man, and sometimes many times that. They come in many varieties, but are most commonly depicted with red or blue skin, wild hair, two or more horns, and fang-like tusks. Other variations exist in different colors and with different numbers of horns, eyes, or fingers and toes. They wear loincloths made of the pelts of great beasts. All oni possess extreme strength and constitution, and many of them are also accomplished sorcerers. They are ferocious demons, bringers of disaster, spreaders of disease, and punishers of the damned in Hell.
Oni are born when truly wicked humans die and end up in one of the many Buddhist Hells, transformed into Oni. They become the ogreish and brutal servants of Great Lord Enma, ruler of Hell, wielding iron clubs with which they crush and destroy humans solely for enjoyment. An oni’s job is to mete out horrible punishments such as peeling off skin, crushing bones, and other torments too horrible to describe to those who were wicked (but not quite wicked enough to be reborn as demons themselves). Hell is full of oni, and they make up the armies of the great generals of the underworld.
Occasionally, when a human is so utterly wicked that his soul is beyond any redemption, he transforms into an oni during life, and remains on Earth to terrorize the living. These transformed oni are the ones most legends tell about, and the ones who pose the most danger to humankind.
These oni are the stuff of legends and fairy tails, countless stories of lords and ladies, warriors and rogues that make up Japanese mythology. No two stories about oni are exactly alike except for one thing: oni are always the villains of mankind.
Originally, all spirits, ghosts, and monsters were known as oni. The root of their name is a word meaning “hidden” or “concealed,” and it was written with the Chinese character for “ghost.” In the old days of Japan, before the spirits were as well-cataloged as they are today, oni could be used to refer to almost any supernatural creature – ghosts, obscure gods, large or scary yokai, even particularly vicious and brutal humans. As the centuries shaped the Japanese language, the definitions we know today for the various kinds of monsters gradually came into being. Today, the word oni generally only refers to this specific category of male demons. Female demons are known by another name: kijo.