The Elemental Sanctuary is a location in The Minish Cap. It is bridged between the Light World and the Minish World. The entrance can only be seen by Hylian children and the Picori, but the entrance only reveals itself once every century.
Link will go to the Elemental Sanctuary after he acquires one of the Four Elements. When Link arrives here, he places the Picori Blade in the pedestal, which makes all the current elements Link has fuse into the Picori Blade making the White Sword. Also, the more elements Link has fused into the sword, the more clones he will be able to make. Once Link gets all the elements, it will forge the Four Sword giving Link the ability to make four clones.