Oni

The Oni (also known as the Bringers of Doom) are a race of demonic beings. 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. It is unknown why they left prior to Season 9. Their current goal is to conquer all the realms, including Ninjago. They serve as the main antagonists of the tenth season, March of the Oni.

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 sent 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.
 * It is also worth noting that the Overlord's "true form" in Season 2 was that of a dragon.
 * 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, known to the Dragon Hunters as Oni Land.
 * Iron Baron and his Dragon Hunters have never seen an Oni, and mistook the Ninja for Oni upon their arrival to the First Realm.
 * The Oni are shapeshifters, just like Boggarts from the Harry Potter book franchise
 * This was revealed by Iron Baron in "Firstbourne" and by Mistaké in "Two Lies, One Truth," when she shows her powers to Lloyd, Nya, and Dareth by transforming into a rat.
 * 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 an impending 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.
 * This theory was confirmed in the trailer for the 10th season.
 * 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 existed 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, were able to live for many centuries.
 * Despite their power of shapeshifting being similar to the element of Form, the Oni do not actually have the elemental power.