Shadow Crystal

The Shadow Crystal is an item in.

Location and Uses
After completing the Lakebed Temple, Link and Midna arrive at Lanaryu's Spirit Spring, where they are confronted by Zant, who curses them both when Midna refuses to help him. Zant summons the Shadow Crystal and, in response, Wolf Link attacks, only to have the crystal forced into his forehead, leaving Link trapped in his Wolf form.

Deep within the Sacred Grove lies the Master Sword, which Link and Midna find after following the Skull Kid. As Link approaches the Sword, he transforms back into his human form. Therefore, the curse of the Shadow Crystal is broken, and it coalesces in Midna's hand.

By using the Shadow Crystal, Link gains the ability to transform into wolf form at will, as well as use Twilight Portals to warp anywhere.

Trivia

 * The Dark Fog that appears in the Palace of Twilight is also made up of Shadow Crystals.