Nobody knows the origin of the Oni, though some suspect they are the souls of the evil and insane dead, brought back to the world of the living from jigoku (hell) by Izanami, Goddess of Death, for purposes unknown.
The basic Oni template is detailed below. The DM should roll for appearance/special abilities and hit dice in the relevant charts.
Organisation: Solitary, Band
Activity Cycle: Any
Intelligence: Low - High (d6+8)
Alignment: Chaotic Evil
No. App.: 1, or 1-8
Armour Class: -1 - 2 (d4-2)
Hit Dice: 5 - 10 (d6+4)
Thac0: By Hit Dice
No. of Attacks: 3
Dam/Attack: d10+2/d10+2/d8+3 (By weapon/by weapon/bite)
Special Attacks: If an Oni is wielding a kanabo, with each successful strike he can stun his enemy, causing the opponent to lose initiative in the next round.
Special Defenses: Oni can only be wounded by enchanted weapons.
Magic Resistance: 1d100%
Morale: Champion (15-16)
Roll one d12 d3 times on the following table for each Oni's appearance/abilities:
1 - Horns. The Oni has two twisted horns sticking out from its forehead, like a goat.
2 - Horn. The Oni has a single, narwhal-like horn sticking out from its forehead.
3 - Skin Like Ice. The Oni's skin is freezing cold, and anything it touches or which is touched by it suffers 1 hit point of cold damage.
4 - Skin Like Fire. The Oni's skin is boiling hot, and anything it touches or which is touched by it suffers 1 hit point of heat damage (this result cancels a roll of '3').
5 - Withering Gaze. The Oni's very glare is enough to chill the soul. Three times per day it can gaze to cause energy drain.
6 - Vampiric Bite. If the Oni hits with its bite attack, it can drain d6 additional hit points and add them to its own total.
7 - Coloured Skin. The Oni has purple (1), red (2), yellow (3) or blue (4) skin (roll a d4). Purple-skinned Oni have +25% resistance to magic. Red-skinned Oni have 50% resistance to fire damage. Yellow-skinned Oni can only be hurt by weapons enchanted to +2. Blue-skinned Oni have 50% resistance to cold damage.
8 - Stinging Beard - The Oni has a long, ragged beard, interwoven with rocks and pieces of metal and broken glass. It can use its beard like a scourge in combat, gaining an extra attack for d4 damage.
9 - Third Arm - The Oni has a third arm, and gains an additional attack per round.
10 - Third Leg - The Oni has a third leg, which raises its movement rate to 15.
11 - Tail - The Oni has a prehensile tail, which can hold a weapon of small size to gain an extra attack.
12 - Third Eye - The Oni has a third eye. This allows it to cast spells as a 3rd level Magic User.