From the Super Mario Wiki
The Thunder Palace is an ancient temple located on Oho Oasis in Mario & Luigi: Superstar Saga. It is inhabited by Thunderhand and serves as a companion to the nearby Fire Palace. When Luigi enters the Thunder Palace during the adventure, Thunderhand warns him to leave, for they have no business with each other. After tThunderhand leaves, Luigi touches the Thunderhand spark, a blue orb located on a pedestal in the room. This infuriates Thunderhand. However, upon noticing that Luigi survived the Spark, Thunderhand decides to teach him the Thunderhand technique. This allows Luigi to produce electricity with his hands.
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