[s11e27] — Corruption

The Ninja pursue Garmadon to the coast. Kai, Jay, Zane, and Cole are forced to stay behind to deal with the newly evil Nya.

Before leaving, the Garmatron is tested on Nya. This results in her being corrupted, turning evil, and gaining Garmadon's dark abilities. [S11E27] Corruption

Here is a helpful summary and breakdown of the events in that episode: The Ninja pursue Garmadon to the coast

The sudden, forced turn of a close ally (Nya) creates high emotional stakes for the team. [S11E27] Corruption