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 the Thunder God and serves as a companion to the nearby Fire Palace. When Luigi enters the Thunder Palace during the adventure, the Thunder God warns him to leave, for they have no business with each other. After the Thunder God leaves, Luigi touches the Thunder Spark, a blue orb located on a pedestal in the room. This infuriates the Thunder God. However, upon noticing that Luigi survived the Spark, the Thunder God decides to teach him the Thunderhand technique. This allows Luigi to produce electricity with his hands.
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