Oni Masks can each summon a different tribe of Shadowkhan once the mask is attached to a host's face, but if all nine masks are brought together in one location, the masks shattered into pieces and the Oni spirits are set free, summoning enough Shadowkhan to engulf the Earth in eternal darkness.

If worn too long the masks will possess the wearer until he/she is consumed by the demon's psyche and corrupts the wearer's chi. Depending on how much of the mask is worn by the host, the Shadowkhan will only have that much of them summoned. The wearer is also granted with superhuman strength, speed and agility to the extent that they could bend prison bars and lift Tohru with one hand. Since they are not full Oni, users are not affected by onions.

Each mask has a different tribe of Shadowkhan and a different removal ingredient. The removal spell also requires a lizard tail, wolfsbane, sulfur root, and left eye of newt.
A tenth mask was forged intended to imprison Tarakudo like the rest of his demon generals but he got to it first and hid it in the Shadow Realm, putting it beyond the reach of any human without the use of shadow magic and the means to locate the mask.

Oni Masks originate from Japan and were created by ancient good chi wizards to seal the Oni Generals within them.

Shadowkhan - Removal Ingredient - Wearer
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Ninja Khan - Japanese Steel - Chow
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Razor Khan - Japanese Silk - Ratso
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Bat Khan - Japanese Rice - Scruffy
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Sumo Khan - Rare Japanese Seaweed - Captain Black
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Samurai Khan - Japanese Cherry Blossom - Finn's backside.
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Squid Khan - Tentacle of Japanese Octopus - Paco
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Crab Khan - Tears of Japanese Monkey - Valmont (left half) Jade Chan (right half)
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Mantis Khan - Unknown - None
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All - Hair of a Samurai (or a descendant of one) - Tarakudo
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Mini Khan - Fish - Hak Foo