Twilight

Twilight is matter originating from the Twilight Realm that plagued Hyrule during The Legend of Zelda: Twilight Princess, 100 years after the events of The Legend of Zelda: Ocarina of Time. The Twilight King, Zant, was granted this power by Ganondorf in exchange for his body being Ganondorf's spirit's host. Twilight covered the land and reduced Hylians to spirits, and gave shadow power to various enemies.

Link and Ganondorf had the ability to change into Twilight and use it to warp. Using Twilight, Ganondorf took over Princess Zelda's body. Eventually, Link restored power to the light spirits of Hyrule, cleansing the land. The twilight was stopped for good with the death of Zant.

Behind the scenes
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. Perhaps it was to make the Twilight Realm seem more "Twilight" by adding the colors of Twilight.