Oni

The Oni (also known as the Bringers of Doom) were a race of demonic beings, led by the Omega. They were 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. Prior to Season 9, the surviving Oni left their realm with the goal to conquer all Sixteen Realms, including Ninjago.

Prior to the series
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. With the help of a kind Oni woman named Mistaké, the child, eventually known as "the First Spinjitzu Master" beat back the pursuing forces. Before their defeat, the Oni left the three Oni Masks behind as a sign of their time in the realm, building a temple deep within Primeval's Eye, where they depicted their origins on the walls and sealed away the Oni Mask of Hatred so it could only be taken by a being with Oni blood.

Thousands of years later, the First Spinjitzu Master's son, Garmadon, would inherit his Oni blood from his father. Once awakened, the blood that flowed through his veins would turn him into a megalomaniacal monster, bent on conquering Ninjago. He would then go on to cause many of the conflicts in Ninjago's history. His evil actions would be manipulated by The Overlord, a manifestation of Darkness born from the shadows to balance out the First Spinjitzu Master's light, in his attempt to restart the "Final Battle" between good and evil, and rise to power over all.

Prior to Season 9
In the present, the majority of the Oni have mysteriously disappeared from their Realm, presumably to begin their conquest of the realms, leaving the Oni Land abandoned and in disrepair, due to being untended to in centuries.

The only full-blooded Oni that was known to be alive is Mistaké, who remained in Ninjago after helping to defend it, now disguised as an elderly tea lady.

After his banishment to the Cursed Realm, and the subsequent drowning of its embodiment The Preeminent at the hands of Nya, Garmadon and the realm's other inhabitants passed on to the Departed Realm. While here, he saw an "endless night" which filled him with fear. Believing only the power of Destruction he inherited from his father could protect his father's realm from the coming threat of the Oni, Garmadon sought out Harumi upon his resurrection, plotting to conquer Ninjago and overcome his love for his son in order to stir up enough conflict to unlock his True Potential.

The Darkness Comes
For reasons unknown, The Oni returned to the First Realm but attacked the inhabitants of Dragon Hunters and Dragons. Everyone touched by the darkness was petrified but Firstbourne and Faith managed to escape and were injured. The Oni then came into Ninjago City through the Realm Crystal. They quickly spread a dark cloud, petrifying everyone in Ninjago City. The Commissioner and his police men were cornered in a alleyway only to be save by the Ninja. The Ninja tried to use their powers on the cloud but their Elemental Powers had no effect. The Ninja retreated, although they struggled to save Cole, who was grabbed by the Oni. Lloyd saved Cole and they returned to the rebuilt Destiny’s Bounty. Cole then said the tentacle of the Oni was the coldest thing he ever touched. The entire city was engulfed in the black cloud with the Ninja barely escaping.

Into the Breach
The Oni continued to spread across the city, petrifying many of the citizens. Garmadon and Lloyd went into the cloud to destroy the Realm Crystal, where they encountered two Oni guarding Borg Tower. After defeating them, they entered the building, but before they could destroy the crystal, the Omega manifested and attacked them.

The Fall
Omega told Garmadon and Lloyd that he wanted to engulf Ninjago in Darkness, so that the power of Creation would be forever gone and Destruction will take its place. The Omega fought Lloyd and Garmadon, and beat the pair back. Garmadon and Lloyd managed to destroy the Realm crystal although it did not stop the Oni Invasion. He then teleported many of the Oni to their location. While on the run, the two managed to find and take the Golden Armor, which had been kept in the Borg Tower. Omega led a army of Oni to stop Garmadon and Lloyd, However, they escaped with the help of P.I.X.A.L..

Endings
The Oni attacked the Monastery at the same time and the Ninja, Wu, Faith and Garmadon held a last stand to protect the citizens taking refuge in it. After the Ninja decided to use the Tornado of Creation to destroy the Oni, Lloyd was Knocked Unconscious and saw a vision of the First Spinjitzu Master. A wave of Golden Power spread across the realms saving everyone who had been petrified. It is unknown what happened to the Oni.

Known Oni

 * Mistaké (Deceased)
 * The Omega

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 were 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, 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, 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 were shapeshifters.
 * 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 seemed 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 did not actually have the Elemental Power.
 * The Oni were able to create dark clouds that feel cold to the touch and petrify those who are trapped within it. Cole said he sensed a presence inside the cloud.
 * A few of the Oni had a face that resembles an Oni Mask
 * They were the only villainous faction with one named member.
 * Golden Power/Creation defeated the Oni in a very similar manner to when Lloyd used the same powers to destroy the Overlord in his dragon form.