The Oni (sometimes Onis) are a vicious race of demonic beings that inhabit the Netherrealm. They appeared in the background for Mortal Kombat and Mortal Kombat Trilogy, and are more prominently featured in Mortal Kombat: Deadly Alliance, with the introduction of the playable Oni Drahmin and the sub-boss Moloch, Mortal Kombat: Tournament Edition, Mortal Kombat: Deception, Mortal Kombat: Shaolin Monks and Mortal Kombat: Armageddon. They also make an appearance in Mortal Kombat (2011) and Mortal Kombat X.
They appear as large monstrous creatures possessing sharp claws, wild hair and usually two long horns growing from the sides of their heads though appear to be mostly human in shape. Despite this being the case, there are some that are more ape-like and burly with glowing yellow eyes. This description is referring to oni like Moloch.
They are well known mostly for their extreme savagery and cruelty, and take pleasure in tormenting the souls of all those unfortunate enough to enter the Netherrealm.
However there is a possibility of onis that were once human beings as in the case of the oni Drahmin that for his crimes in life, he was exiled to the Netherealm where his body and soul was to be tortured for all eternity. Each passing year, the unfortunate soul falls further and further into madness to surrender to the power of that infernal site and eradicate all traces of what was once human.
Oni are said to roam the wastelands of the Netherrealm, always on the search for souls to torment for their own sick pleasure. Those that dwelled on Shang Tsung's Island appeared to demonstrate cannibalism by eating their dead.
In Mortal Kombat: Shaolin Monks there is a hierarchy of Onis. Larger and more powerful members of the Oni are known as Oni Warlords, which act as bosses faced by both Liu Kang and Kung Lao in Goro's Lair and in the Wastelands.
Though most Oni inhabit the Netherrealm, some roamed within Shang Tsung's Island in Earthrealm, in Goro's Lair where you can see their glowing eyes in the dark dungeon, and Outworld's Living Forest where they consumed any unwary trespassers.
Considering the different appearances of Oni, it is possible that there are either multiple species or most have different appearances from one another.
- The term Oni comes from Japanese folklore, similar to Western demons or ogres. They are popular figures in art, literature and Japanese theater. Given that the Outworld races are only loosely based on real-life Eastern culture, it is no different with the Oni, whose common description outside of MK is as strong, horned humanoid creatures dressed in tiger pelts and who sometimes wield massive clubs (kanabō) as weapons.
- Quan Chi was once an Oni, until he became a demon with his mastery over sorcery.
- The common Oni in Deception is dressed similiarly to Quan Chi's appearance from Deadly Alliance onward.
- Oni are assumed to reside only within the Netherrealm but in Mortal Kombat: Shaolin Monks, they are also found in Earthrealm and Outworld.
- The first playable Oni was Quan Chi, however he is only playable in human form. As such, the first playable Oni in its true form was Drahmin.
- An unnamed Oni also makes a playable appearance in the mobile version of Mortal Kombat X.