Thundarr gets hit by the Negative Lightning.

Negative Lightning as a form of energy often associated with Nega-Storms. During Master of the Stolen Sunsword, the Nega-Sword was built by Yando to harness the power of Negative Lightning.

The energy of Negative Lightning has detrimental effects on the Sunsword. When Thundarr was hit with Negative Lightning, it drained the power of the Sunsword and Ariel had to show Thundarr how to recharge it.