When all eight Triforce fragments have been collected, you got the right to go to the "Skull" labyrinth, a.k.a. Death Mountain.  You must go all the way on the other side of the overworld, and near the northwest corner of the overworld there should be a cluster of two rocks.  Bomb the right cluster for a door.  That door is the entrance to Skull labyrinth.

West of Death Mountain, there should be another door.  Enter that door.  That cave is a medicine shop.

Make sure Link is at full life with two medicine bottles and at least 50 rupies before entering the Skull Labyrinth.  Link should have 16 bombs on supply, too.