Black Barrier
The Curtain of Twilight is the giant wall of twilight that cascades over Hyrule in Twilight Princess. It 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 but Link, as the chosen hero, is transformed into a blue-eyed wolf.
“
Last time, a shadow beast pulled you through the curtain of twilight... But if you want to go that way this time, you'll need the cooperation of someone FROM the twilight...like me!
”
— Midna