requirements: light arrows twinmold's remains 16 heart containers hookshot hero's bow and arrows zora mask

to get there go to ikana canyon. there is a river flowing at the bottom of the canyon. use the zora mask and swim upstream (against the current) untill you go through a cave. there will be a waterfall and a cave behind it.

WARNING, the following is a walkthrough of the level, don't read if you want to do it yourself.

when you enter use a light arrow on the sun mark above the door. you can use a magic bean, but you will just get money.

in the next room, you will be challenged to heal the souls of 4 creatures. go into the room on the left.

here you will fight 3 lizard-like creatures from the swamp level. defeat them and you will recieve 100 rupiees in a chest.

now leave that room. go onto the center platform and turn left. there will be 2 paths, take the left path. kill the wizard-like guy inside (the one with all the teleporters) and claime your 100 rupiees.

now leave and go onto the middle platform. turn around and take the right path this time. kill the giant eyeball with the bubbles (just like in the water level [hookshot the bubbles, kill them, then hit the eye with the hookshot/arrows]) kill it then claim your prize of (you guessed it) 100 rupies.

now go down the last path and kill the creature that has the robes and flaming swords (like in stone tower) and get your 100 rupiees, blah blah blah...

now go out and claim your ultimate prize!

hope i could help!