Great Deku Tree: The boy without a fairy... Bring him to me. Go, Navi!
‐ Volume 1; Ocarina of Time - Chapter 1
One dark and lonely night as Link is drifting off to sleep, wondering about the world beyond the forest, he is woken up suddenly by a fairy. Introducing herself as Navi she tells him that The Great Deku Tree sent her. Link's initial reaction is excitement that a fairy finally came to him which Navi is less than enthusiastic about. Shortly after though Link soon learns the reason why she was sent, after a somewhat amusing backfire on one of Mido's pranks that resulted in Link falling right on top of him. As they noticed the plants beginning to whither Navi explains that something evil has entered the forest and that The Great Deku Tree is in trouble and needs Link's help.
Upon their arrival they learn that The Great Deku Tree has been cursed by a creature known as Gohma the parasitic armored arachnid queen. Through awkward teamwork Link and Mido defeat Gohma and break the curse on The Great Deku Tree. Unfortunately he tells them that he will die anyway, but not before telling Link something important. He reveals that Gohma was also cursed and the one responsible for it was an evil man from the black desert whose plan was to capture the Triforce, a legendary relic of the gods that has the power to remake the world into the image of the heart of the one who touches it, and rule Hyrule. If this man were to touch it the world would become consumed by evil. The Great Deku Tree then asks Link to keep this from happening. Although Link feels intimidated by the thought, his words of encouragement give Link the drive he needs in order to embrace his destiny.
Link: Why me? I can't fight against something so scary.
Great Deku Tree: Yes, you can! Learn about the outside world and grow big, Link.
‐ Volume 1; Ocarina of Time - Chapter 2
With his dying breath The Great Deku Tree gives Link the Kokiri's Emerald, the stone that the man was after, and to take it to the princess at Hyrule Castle. He then asks that Link make a shield from his remains which will protect him from evil. After he does this Mido allows Link to keep the Kokiri Sword he used to carve the shield and yells at Link to never return. Although Link is saddened by this, and even perhaps a little reluctant to do so, he leaves the forest. It is here that his unexpected adventure begins, and his chance to see the world.