Twilight

Twilight is matter originating from the Twilight Realm that plagued Hyrule during The Legend of Zelda: Twilight Princess, about a century after the events of The Legend of Zelda: Ocarina of Time. The self-proclaimed Twilight King, Zant, was granted power over twilight by Ganondorf in exchange for his body being used as Ganondorf's spirit's host. Twilight covered the land, reducing Hylians to spirits and giving shadow power to various enemies. Eventually, Link restored power to the Light Spirits of Hyrule, cleansing the land. The twilight stopped completely after the death of Zant.

When a normal person enters the twilight, he or she, if not a twilight creature or any other evil monster, is usually morphed into a spirit. However, the Hero of Twilight takes shape as a "divine beast" to signify he would be the one who would save the two worlds, the Twilight Realm and the Light World.

Midna makes it clear that in order to enter a place where twilight lurks, you must be a creature of the twilight or have a twilight creature take you inside it. Midna also states that not many people can leave the twilight easily.

When a person is transformed by twilight, according to Light Spirit Ordona, it is usually not possible to regain their true form. However, awakening the Light Spirit in that region will accomplish the task.

Twilight creatures use the aid of portals to transport in and out of the twilight. Midna, Ganondorf, and Link, to a certain extent, have the ability to change into twilight and use it to warp. Using twilight, Ganondorf took over Princess Zelda's body.

Beta
The original concept of twilight was vastly different from the twilight that made it in the final version of Twilight Princess. Instead of being yellow and golden the twilight was originally a colorless realm of black and white. Only the characters themselves had any color. For whatever reason, this was changed when the game was delayed in the summer of 2005.