Door of Time
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The Door of Time is located in the Temple of Time and blocks access to the Pedestal of Time, which holds the Master Sword and is the key to entering the Sacred Realm.
The door can only be opened by bringing together three Spiritual Stones: the Kokiri's Emerald, the Goron's Ruby and the Zora's Sapphire. When these three stones are brought together and the Song of Time is played upon the Ocarina of Time at the altar, the door slides open and the path to the legendary blade is revealed.
According to Rauru, the Door of Time is, effectively, an obstacle to the Sacred Realm put in place when the Triforce was sealed away. At the start of Ocarina of Time, Ganondorf is attempting to remove this obstacle but is frustrated by the guardians of the Spiritual Stones.