Dark World

A Link to the Past
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 is referred to as the Light World.

Four Swords Adventures
The Dark World is the fifth level in Four Swords Adventures. It consists of three stages: The Lost Woods, Kakariko Village, and Temple of Darkness.