The Oni (also known as the Bringers of Doom) are a race of powerful demonic beings, led by the Omega. They are the first evil to ever come into existence, as well as the creators of the three powerful Oni Masks. They once inhabited Oni land, a forbidden part of the Realm of Oni and Dragons, the first realm in existence. Oni were at war with the dragons until the First Spinjitzu Master was born. Hundreds of years prior to Garmadon's resurrection, the surviving Oni left their realm with the goal to conquer/destroy all realms, including Ninjago.

The Oni's invasion was halted by the combined efforts of the ninja and the sons of the First Spinjitzu Master, and the demons were vanquished by the Tornado of Creation. Their ultimate fate is left ambiguous.