The Battle of Stiix was a conflict that took place some time after the Second Serpentine War in the city of Stiix. The battle was fought with the Ninja and their allies going up against Morro and the prisoners of the Cursed Realm.
After Morro claimed the Realm Crystal from the Tomb of the First Spinjitzu Master, he traveled to Stiix and used the artifact to open a portal to the Cursed Realm, allowing an army of spirits to spread throughout the city. After Stiix was taken over by the ghosts, the Ninja were forced to sneak in, hoping to destroy the Realm Crystal and get Nya to Morro so she could fight him and the ghosts with her element.
Soon enough, a battle broke out, with the Ninja fighting the other ghosts while Lloyd and Nya fought Morro. As Cole, Kai, Jay, and Zane battled the ghosts, Lloyd had an opportunity to destroy the Realm Crystal but Morro gave him second thoughts, as it was the last means of Lloyd ever seeing his father in the Cursed Realm. Lloyd's hesitation allowed The Preeminent to emerge from the realm and into Stiix.
The Ninja began their next part of the battle against The Preeminent, whose presence prompted Wu to begin evacuating the civilians on a steamboat. However, the ghost warriors began taking possess countless buildings in the City of Stiix, giving her a relative immunity to water and greater mobility. She then proceeded to test out this new tolerance before chasing after several citizens on a fleeing ferry in order to devour their souls.
After saying goodbye to his father, Lloyd rejoined the battle, fighting Morro and traversing several realms while the ghosts created a giant suit for The Preeminent using the buildings in the city. While Lloyd obtained the Realm Crystal, Morro got his hands on using it to return to Ninjago while stranding Lloyd in an unknown realm.
Back in Stixx, as the Ninja fled with the citizens on board the steamboat, The Preeminent waded out into the Endless Sea, eventually seizing and crippling the ship before devouring the souls of those unfortunate enough to be caught by her.
At that moment, Nya found the will to unlock her True Potential, summoning a massive tidal wave that toppled over The Preeminent and sent it sinking into the sea. Morro was seized by one of the beast's tentacles, but his hand was grabbed by Wu, who told him to hang on. However, seeing the error of his ways, Morro rejected Wu's offer to help him, instead giving his former sensei the Realm Crystal before bidding him farewell and sinking into the sea and dying. Along with Morro, all the other prisoners of the Cursed Realm died, including Garmadon, Master Chen and the Anacondrai Cultists (though Clouse somehow escaped).
Though the Ninja saved Stiix, and would soon gain fame for their triumph, the city was left heavily damaged primarily due to The Preeminent; this also resulted in the citizens resenting the Ninja and blaming them for Stiix's destruction.