Oni

The Oni are a race of demons. They are the first evil to ever come into existence, as well as the creators of the three powerful Oni Masks. They inhabited the Realm of Oni and Dragons, the first realm in existence. It is unknown what the whereabouts of the majority of them are by the time of Season 9. They lived in Oni Land, a forbidden part of their realm.

History
Before the existence of any other realms, there was the Realm of the Oni and the Dragon. In this realm existed two races; the Oni and the Dragons. The Dragons had the power to create, while the Oni had the power to destroy, and so they waged a never ending war against each other. One day, however, a child was born of both the worlds of Oni and Dragons, having both the powers of Destruction and Creation. When the Oni and the Dragons fought over which side the child should choose, the child left their world to start a new one, called "Ninjago."

Soon afterwards however, the Oni send some of their forces after the child to his new realm of existence to bring him back to the Realm or kill him, creating a temple deep within Primeval's Eye, where they depicted their origins on the walls of the temple. They would also seal away the Oni Mask of Hatred within the temple, which could only be taken by a being with Oni blood.

Thousands of years later, in the present, the majority of the Oni have disappeared and it is unknown what happened to them. A city of the Oni was found in disrepair, having been abandoned for centuries. The only full-blooded Oni that was known to be alive is Mistaké, who lived in Ninjago shapeshifted into human form.

Known Oni

 * Mistaké (deceased)

Hybrids

 * First Spinjitzu Master (of Dragon descent; deceased)
 * Garmadon (of Dragon and human descent)
 * Lloyd Garmadon (of Dragon and human descent)
 * Wu (of Dragon and human descent)

Trivia

 * The Oni are based on Japanese folklore.
 * The Oni are the first evil to exist in the realms. However, because of Mistaké, it's known that not all Oni are evil.
 * This makes the Oni the first dark beings in the Sixteen Realms, predating the Overlord. However it is unknown if the Overlord has any links with the Oni, due to being the original evil of Ninjago, who rose from the shadows to balance out the First Spinjitzu Master's light.
 * According to a Ninjago magazine, the Oni have mysteriously disappeared from the Realm of Oni and Dragons.
 * In "Two Lies, One Truth," it is revealed that there is a specific area of the Realm of Oni and Dragons that the Oni lived in.
 * Iron Baron and his Dragon Hunters had never seen an Oni, so they mistook the Ninja for Oni.
 * According to the Iron Baron, Oni are shapeshifters.
 * This was later confirmed in "Two Lies, One Truth," where Mistaké shows her powers.
 * Lord Garmadon's initial corrupted form resembles an Oni, lacking some notable features, such as horns, faun legs, and more exaggerated fangs. This is possibly because when he was corrupted, the Devourer's blood made his Oni side stronger, and being struck by lightning brought out the Oni side of his appearance. He's likely missing certain features because he is still only a hybrid.
 * Garmadon mentioned that a darkness would soon consume all of Ninjago. At the end of "Green Destiny," Lloyd concluded that it was the missing Oni who would be the darkness.
 * If Lloyd is correct, then it is unknown how the Oni entered the Departed Realm.
 * If Garmadon was being truthful and if Lloyd's speculation is correct, then it seems that the Oni may be the antagonists at some point in 2019.
 * In the Russian, and some other Slavic languages, the word "they" translates to "oni," which adds to the theory that Garmadon was referring to the Oni at the end of "Green Destiny."
 * The Oni also seem to be extremely long-lived, as the First Spinjitzu Master, a half Oni, was able to live for several thousand years and Mistaké even longer. Even those very distantly related to the Oni seem to have that power, as Garmadon and Wu, who are only a quarter Oni, are able to live for many centuries.
 * Despite the power to shapeshift being similar to the element of Form, the Oni do not actually have the element.