The Oni Temple was a temple that was constructed by the Oni when they arrived to Ninjago in search of the First Spinjitzu Master. Located deep within Primeval's Eye, the temple is where the Oni Mask of Hatred, the third and final Oni Mask, was kept.
Sometime after the First Spinjitzu Master fled from the Realm of Oni and Dragons and created Ninjago, the Oni pursued him, creating the Oni Temple, where they carved their origins onto the walls of the temple. They also stored the Oni Mask of Hatred within the temple, which could only be obtained by a being with Oni blood within them.
Armed with a map to the temple, Lloyd and Harumi journeyed across Primeval's Eye in search of the temple. After falling down a waterfall and reaching land, Lloyd discovers the Oni Temple hidden between the waterfalls.
Lloyd and Harumi manage to locate the Oni Mask of Hatred within the Oni Temple after passing several obstacles. Harumi attempts to grab it, but a force makes it so she cannot. She then explains to Lloyd that someone with Oni blood can obtain the mask. After Lloyd takes the mask, he soon begins to question Harumi on how she knew that he was part Oni. After a moment of revelations, Lloyd discovers Harumi's true intentions and realizes that she is the Quiet One. A fight then ensues, with it ending with Lloyd throwing the Oni Mask of Hatred over the side of the platform. However, Harumi dives after it, catching it while she manages to use her knife to hold onto the side of the platform.
Armed with the Oni Mask of Hatred, Harumi puts it on, using its powers to jump back onto the platform. Lloyd questions what the mask did to her, where she reveals that it has made her invincible to all forms of damage. After a very brief fight, Harumi reveals to Lloyd that she wants him to feel the pain she has felt following the death of her parents by having Lord Garmadon destroy Lloyd. Harumi then proceeds to flee from the temple with the mask, utilizing her invincibility to destroy the temple in an effort to destroy Lloyd. However, Lloyd is able to escape in time.