In ancient times, Tarakudo, lord of all Oni and king of the Shadowkhan, along with his nine demon generals, terrorized Japan. Ancient Chi warriors carved Oni Masks and through use of good Chi spell ingredients, trapped the Generals in those masks.
One was made for Tarakudo who found the mask and hid it in the Shadow Realm. His whereabouts were unknown afterwards as he was dormant for centuries. The masks were then scattered across the Earth. If united, the nine Oni generals would be released and summon an army of Shadowkhan to engulf the planet in eternal darkness which will cause all good magic to erode.
At an unknown point in time, Shendu found a mask and means to tap its power indirectly, letting him summon the Ninja Khan without wearing it. This same mask's powers were later briefly accessed by Jade when she tattooed the symbol of Tarakudo on her leg.
After Shendu was banished into the Netherworld, Daolon Wong used a spell to draw power from the very same mask. However, his magic powers were taken away and he was no longer able to control the Shadowkhan.
The mask was kept in Shendu's Place in Hong Kong until a freshly awaken Tarakudo and the Enforcers found it.
Ultimately, all ten Oni were sealed away within Tarakudo's mask.