Dark World

In A Link to the Past, the Dark World is a corrupted version of the Golden Land after Ganon reaches it and takes the Triforce (which happened in Ocarina of Time). It is known that the Golden Land is a mirror that reflects a person's true self, so Ganon and his minions are turned into monsters when they enter the Golden Land, and Link turns into a rabbit unless he has the Moon Pearl. When Ganon takes the Triforce, the whole Golden Land changes, becoming as evil as his heart, being known as the Dark World. When that happens, Hyrule becomes the Light World.