Millions of years ago, Three Goddesses descended upon the Chaos that was Hyrule. Din. With her flaming strong arms created the Red Earth. Nayru. Poured her Wisdom on the Earth and created the Spirit of the Law. Farore. Created all life forms that would uphold the law.  (Basically, they created the world and stuff, but this way sounds much more atmospheric and metaphorical don't' you think?) When the Goddesses left Earth to return to the Heavens, they left behind the Triforce. Since then, the land the Triforce stood on has become sacred land, hence the Sacred Realm. Over the years there have been wars to claim the Sacred Relic. Many Years ago a Woman gravely injured rode away from the Castle towards the forbidden forest and entrusted her Son to the Great Deku Tree. I think that is how it goes, anyway let's see how things are turning out for our Heroes.