Here is a helpful summary and breakdown of the events in that episode:
Lloyd, Misako, and Sensei Wu rush ahead to stop the Garmatron from reaching the coast. Key Themes [S11E27] Corruption
Garmadon successfully completes the Garmatron, a massive cannon capable of spreading darkness. He, along with the Overlord and the Stone Army, drives the vehicle toward the coast of the Dark Island. Here is a helpful summary and breakdown of
The Ninja pursue Garmadon to the coast. Kai, Jay, Zane, and Cole are forced to stay behind to deal with the newly evil Nya. [S11E27] Corruption
The sudden, forced turn of a close ally (Nya) creates high emotional stakes for the team.