The Legend of Zelda
A Link to the Past
The Fat Faerie in the Dark Pyramid will give Link the only weapon which is capable of defeating Ganon: the Silver Arrows."
BS The Legend of Zelda
In the final fight, once Ganon is stunned with the hero's sword, he can only be finished off with a Silver Arrow.
The Silver Arrow is found in Level 8 of MAP1 and MAP2. To obtain it, the hero must allow themselves to be captured by green Wallmasters in the centre room of the north-most row (an Old Man in the room to the right hints at this, saying "The ones next door can bring you to the arrow."). Once back in the entrance room, pushing a block (centre-right in MAP1, top-left in MAP2) will open the stairs to the arrow.