From Zelda Legends Wiki
When Ganondorf got the Triforce, his wish to conquer the world changed the Sacred Realm into the Dark World. The Dark World is a twisted reflection of Hyrule, the Light World. Sometimes events in one world affects what happens in the other world. Entering the Dark World turns you into a shape that reflects what is in your heart.
The name "Dark World" refers to the dark, twisted copy of Hyrule ruled by Ganon in ALttP. The Dark World was once the Golden Land, or Sacred Realm, before it became twisted by Ganon's evil. The Dark World is like Hyrule, only darkened and twisted by Ganon's evil might. It is possible to enter the Dark World through eight warp tiles scattered throughout Hyrule, but impossible to escape back into Hyrule. This is because the Seven Wise Men sealed the way back to Hyrule long ago, during the Imprisoning War. However, the Magic Mirror can circumvent the Seal, allowing the user to travel back to Hyrule at will. All those who enter the Dark World are changed by the Triforce into a form that reflects what is in their heart. Only the Moon Pearl can counter this powerful magic.
It is unclear exactly when the Golden Land, or Sacred Realm, was changed into the Dark World. The ALttP manual tells of the Imprisoning War, in which Ganondorf entered the Golden Land, stole the Triforce, and tainted the Golden Land with his evil power. Even when Ganon's army invades Hyrule, the manual tells that "Ganon's evil army swarmed from the tainted Golden Land." Furthermore, the Seven Wise Men are able to "magically seal Ganon in the Golden Land." Centuries later, the King orders an investigation of the "Imprisoned Dark World (as the Golden Land had come to be known)." It would seem, then, that the Dark World was not created until after Ganon was sealed in the Golden Land.
Standard historical interpretation places the real events of the Imprisoning War during OoT. A legend told by Zelda in OoT states that "the resting place of the sacred triangle, the Sacred Realm, is a mirror that reflects what is in the heart... the heart of one who enters it... If an evil heart, the Realm will become full of evil; if pure, the Realm will become a paradise." According to Zelda, "the Sacred Realm became a world of evil" before Ganondorf invaded Hyrule. The Sacred Realm is also called a "void" several times, and near the end of the game it is called the "Evil Realm" by Rauru and Zelda. Note how Rauru commands: "Now, open the sealed door and send the Evil Incarnation of Darkness into the void of the Evil Realm!!" And Zelda tells Link, "Thanks to you, Ganondorf has been sealed inside the Evil Realm!"
So two things are clear. First, the Sacred Realm became filled with evil as soon as Ganondorf got the Triforce, and became known as the Evil Realm, a void, after Ganondorf invaded Hyrule. Second, the Golden Land is not known as the Dark World until long long after OoT. Given these facts, there are several possible interpratations of when the Dark World first came into being. It can be argued that the Sacred Realm became the Dark World as soon as Ganondorf got the Triforce in OoT. But then it becomes difficult to explain why the Evil Realm is called a "void." Another theory would solve this difficulty by saying that the Sacred Realm became a world of evil in OoT, but not the Dark World. Ganondorf brought all of his evil with him into Hyrule when he invaded, leaving the Evil Realm a void. The Dark World is only created after Ganondorf is sealed back in the Sacred Realm. But this leaves a problem: did Ganon simply create the Dark World out of nothing? Surely there were some raw materials that he used. This is why some theories argue that the entire OoT alternate future was sealed in the Sacred Realm. After all, not only Ganondorf is sealed; according to Rauru, "the Sages' Seals will contain all the evil power in the void of the Realm."
There is often confusion about why people change form upon entering the Dark World. Within the game, it is generally agreed that the Triforce's magic changes people into a forms that reflect their hearts. But it is unclear whether this process is an inevitable side effect of being in the Triforce's presence, or whether Ganon is using the Triforce to do this. The issue will probably always remain in dispute, because the games never specify what level of control Ganon has obtained over the Triforce. My opinion is that the whole Dark World, and all the effects of entering it, are caused by Ganon. After all, it is stated in OoT that "the Sacred Realm is a mirror that reflects what is in the heart," not the other way around. But it is possible to argue that the transformation is an inevitable side-effect of entering the Golden Land, where the Triforce rests. In fact, the old man on Death Mountain says that "the Moon Pearl will protect its bearer from the magical air of the Golden Land, so you can keep your human shape there," implying that the very air is what changes people. But I would still argue that Ganon is using the Triforce to cause the Dark World to have this effect on people.
Relationship to Hyrule
Why is the Dark World like a copy of Hyrule? According to the Essence of the Triforce, "Ganon's wish was to conquer the world. That wish changed the Golden Land to the Dark World." The Dark World is representative of what Ganon wishes to do to Hyrule. The "Light World" is a synonym for Hyrule, and the Light and Dark Worlds are referred to as two different worlds, or two halves of the same world. The two worlds are connected in some way. Objects exist simultaneously in both worlds, in slightly different forms. Changing something in the Light World sometimes changes its twin in the Light World. (This only seems to work in the Swamp Palace). In some theories, this may have something to do with the past and future in OoT becoming the Light and Dark Worlds. Hyrule is also referred to as the "real world," and the Dark World as a twisted copy of it.
ALttP is the only game that refers to the "Dark World" by that name. Thus, all relevant quotes listed below come from the ALttP manual and game. For more information about the Dark World's origins, see the references listed below, especially the Sacred Realm, Golden Land, and Evil Realm.
- "The king of Hyrule, after counsel with his sages, ordered an investigation of the Imprisoned Dark World (as the Golden Land had come to be known) but the wise men's seal was apparently intact." (p.6)
- "The Golden land of the Triforce has become a Dark World corrupted by Ganon's evil wishes." (p.8)
- "Hyrule (the Light World) and the Dark World look alike but have subtle differences. Find the difference between the two maps, and you can solve many mysteries of both worlds." (p.22)
- "People who enter the Dark World are transformed into a shape that reflects what is in their hearts. You can prevent this by obtaining the Moon Pearl, which protects its bearer from the power of the Triforce." (p.24)
- "There are eight magical tiles which are entrances to the Dark World that was sealed by the Wise Men long ago." (p.30)
- Old man on Death Mountain:
The Moon Pearl will protect its bearer from the magical air of the Golden Land, so you can keep your human shape there.
Note: he says "Golden Land," but it's pretty obvious that he's referring to the Dark World.
Surprisingly, the Triforce created this world to fulfill Ganon's wish. What is Ganon's wish, you ask? It is to rule the entire cosmos! Don't you think it might be possible with the power of the Triforce behind you?
- Ball on Dark World Death Mountain:
This world is like the real world, but evil has twisted it.
The place where you now stand was the Golden Land, but evil power turned it into the Dark World." ... "In order to save this half of the world, the Light World, you must win back the Golden Power.
The power of the Dark World quickly turned me into this tree shape...
- Maiden in Village of Outcasts:
As the wise men sealed the way to the Dark World, the Knights Of Hyrule defended them from the attacks of evil monsters.
- Sahasrahla, in Swamp Palace:
Objects exist simultaneously in both worlds with similar shapes. If the form of a thing changes, it will affect the shape of its twin in the other world.
- Maiden in Swamp Palace:
There are some other magical warping points like the one you saw on Death Mountain. By using them you can go between the two worlds and find the evils hidden in the Dark World.
I will destroy you and make my wish to conquer both Light and Dark Worlds come true without delay.
- Essence of the Triforce:
Ganon's wish was to conquer the world. That wish changed the Golden Land to the Dark World." ... "But now, you have totally destroyed Ganon. His Dark World will vanish.