Black Barrier

The Curtain of Twilight is a location in. It is the giant wall of Twilight that cascades over Hyrule.

Features and Overview
The Curtain of Twilight is only penetrable by having someone in the Twilight Realm grab Link and drag him in. When seen from afar, the entrance appears to be but a black wall with golden outlining, but when observed at a closer distance, strange markings are seen written on it. Any non-Twili passing into the Twilight without the protection of the Master Sword will be affected by its power and transformed, in most cases, into spirits. Although Link, as the chosen hero with the Triforce of Courage, is transformed into a blue-eyed wolf upon passing through it instead. When the Twilight is lifted from the province covered by it, the Curtain of Twilight disappears along with it.

Trivia

 * In an unused text string, Midna admits that Link can enter the Twilight even without her help, and apologizes for lying to him.