Found in the "Tech Shop" in the back of the mall. You must solve a wire puzzle: red to red, blue to blue. The twist: If you short-circuit it (cross the wires), you get a "Cursed Battery" which unlocks a hidden boss later but locks you out of the good ending. Do not cross them.

You’ll enter a mine or a cave eventually—maybe by falling, maybe through a hidden door. The music changes. Enemies become whispers.