Twilight

"The dark clouds of dusk cover this land... This drape of shadows is called...twilight. In this twilight, those who lived in the light...become as mere spirits...it is a place...where the dark ones and evil creatures dwell..."

- Faron

Light in Hyrule is protected by the four great Spirits of Light, who each support the light in their respective province. During the events of Twilight Princess, that light is taken away, and the provinces from which the light is stolen succumb to the Twilight as it's light fades away. Zant, the Twilight King, steals the Light from the Light Spirits, and the kingdom of Hyrule is slowly overtaken by Twilight as it's light vanishes.

Once overtaken by the Twilight the inhabitants of the Light are turned into lost spirits. They lose their physical bodies, and become spirits. They are for the most part, unaware of the events, but can see the shadow creatures and other actions performed by them. Once someone has entered the Twilight, it is often impossible to escape or recover. Only with the help of a member of the Twili race can a lost soul or anything trapped in the Twilight escape. However, there are a select few that can survive in the Twilight without becoming a lost soul. Bearers of the Triforce can usually withstand the effects of the Twilight. Link, bearer of the Triforce of Courage, has been blessed by the Gods to survive the effects of the Twilight Realm. So instead of becoming a lost spirit, Link retains a physical form, however the form is that of a wolf.

"Twilight covered Hyrule like a shroud, and without light, the people became as spirits. Within the twilight, they live on, unaware that they have passed into spirit forms... All the people know now is fear... Fear of a nameless evil..."

- Princess Zelda

Not all beings are turned into spirits however. Many are turned into "shadow creatures" within Twilight and become mere shadows of their former selves, no hint of personality. For example, Bulblins that get caught within Twilight changed to twisted versions of themselves, with grey helmets and black/grey skin tones like the Twili have. Keese, Deku Babas and Kargaroks are just a few creatures that change within the Twilight Realm. Shadow creatures also have the ability to change people or spirits into monsters. As told in Twilight Princess, an old lady goes into the General Store in Kakariko Village and is transformed into a shadow creature.

Twilight, however, can be pushed back and eventually destroyed. This is done by returning the Light to the respective province by giving back the light to the Light Spirits. Once the light is returned to the Spirits, light returns to the respective province and the Twilight vanishes.

See also: Twilight Realm