Spiritual Stone
In order to open the Door of Time which seals away the Master Sword in the Temple of Time, one needs the three Spiritual Stones.
The three stones are the Kokiri Emerald (Spiritual Stone of Forest), held by the Great Deku Tree, the Goron's Ruby (Spiritual Stone of Fire), a treasure held by Darunia, and the Zora's Sapphire (Spiritual Stone of Water), which is held by Princess Ruto.
Ganon was refused the three stones by their guardians. In response, he plagued them with various blights. In order to obtain them, Link must free the guardians from Ganon's curses.