The Sacred Realm is more of a holy grounds on the Zelda world and there is much scepticism that comes along with it but I will start off with the basics, when the three goddesses created the land of Hyrule they all granted different aspects to the land and when they had completed the god like quest the returned back to whence they came. They all returned in a certain spot in the air (to be quite literal) and they have forever opened a rift in the world of the sacred realm and the world of man. The market (in Oot) has a cathedral in which you may enter the sacred realm in a certain point of the game so to all understanding that is where the goddesses departed. Also the cathedral is where you obtain the Master Sword but in the Legend of Zelda Twilight Princess you obtain the Master Sword in the Sacred Grove. Some believe that the Sacred Grove is where the cathedral once stood and that is a possibility but many would have to look back at the multiple and possible time lines of Zelda to absolute.
The Sacred Realm is the one place evil cannot reach and for all the games that it has stood in part its title has stayed true.
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