The Oni are the overarching antagonistic faction of the Oni and Dragon Trilogy from LEGO Ninjago: Masters of Spinjitzu, serving as the unseen overarching antagonistic faction of both Season 8: Sons of Garmadon and Season 9: Hunted, as well as the main antagonistic faction of Season 10: March of the Oni.
They are a mythological ancient race of demons who wish to fill Ninjago with darkness and destruction they are the sworn enemies of the dragons.
The Oni came into existence many years ago when the entire universe was created. They were a vile and destructive race and when the dragons were born the oni went to war with them but one day a child was born the child was part oni and part dragon the two sides wanted the child to join one of their races against the other however the child refused grew up and created the world of ninjago it is presumed that while the First Spinjitzu Master created ninjago and fought the Overlord and the stone army the oni and dragons were still at war with each other the first spinjitzu master then had two sons named Wu and Garmadon and he told them about their heritage the oni created the masks of vengeance deception and hatred after the defeat of the sons of garmadon and the Iron Baron the oni had begun their plan to bring darkness and destruction in ninjago with the ninja opposing them Lloyd Garmadon and his father went to a building to try to stop the oni's plans but they unfortunately encountered the oni leader the Omega who was too powerful for them to beat the oni leader then introduced himself as the Destructor and the Bringer of Doom and asked the two if they protect the realm Lloyd said yes and the Omega then asked Garmadon if he protected the realm as well the dark lord responded no but later said he protected the realm for the time being the Omega tells Garmadon that he senses oni blood in him and asks him why does he protect the realm Garmadon then says that if any evil entity will conquer ninjago it will be him the Omega then responds by saying that Garmadon will also perish along with the realm he and the ninja protect later the oni go to attack the monastery of spinjitzu where the ninja and their allies are waiting for them some of the oni battle the ninja while the Omega fights with Garmadon while fighting Garmadon the oni leader taunts him despite their numbers some of the oni get defeated by the ninja but more oni keewhat p coming the ninja with no more plans left decide to use the tornado of creation to defeat them although reluctant at first Garmadon decides to join once he sees a picture of Lloyd get covered by darkness the oni break down the monastery doors only to discover the golden light the tornado makes and get devoured by the light it is unknown what happened to the oni after they were hit by the tornado it can be implied they were either destroyed or they disappeared.
All of the Oni seem to have been ruthless and destructive creatures not caring for the lives of other beings they also appear to be very chaotic as they prefer destruction over creation the oni also greatly despise the dragons going as far as to start a war with them which may have lasted for many years until they went to spread their chaos in Ninjago however some oni aren't evil as Mistake was a good oni and helped the ninja on a few occasions before her death what is also interesting about the Oni is that most of them do not speak simply growling or making other noises.