Mirror of Twilight

In The Legend of Zelda: Twilight Princess, the Light Spirit Lanayru tells Link of a story, about Dark Interlopers, who tried to take control of the Sacred Realm, and the Triforce. The three light spirits, Faron, Eldin, and Lanayru stopped them and inprisoned them in the Twilight Realm. They needed to leave sort sort of way to communicate between the two realms, and so the Mirror of Twilight was created.

Story

After Link collects all the fused shadows, he goes to Arbiter's Grounds to find the Mirror of Twilight. Midna wanted to go in order to find Zant. However, Link and Midna discover that Zant has broken the Mirror of Twilight into four pieces, and thus must go look for them. The first piece is at the top of Snowpeak, the other in The Sacred Grove, and the final piece lays in The City in The Sky.