Original: As the battle escalates, Iroh watches Zhao capture the moon spirit Tui, which with its water spirit counterpart had assumed the form of a fish. Despite Iroh's pleas, Zhao kills Tui to strip the Waterbenders of their powers and abilities to Waterbend.
Simplified: As the battles escalates, Iroh watches Zhao slay the moon spirit Tui, causing the Waterbenders to lose all of their powers and abilities to Waterbend.