Shadow Crystal

From Zelda Wiki.org

The Shadow Crystal is a crystal of dark magic in Twilight Princess.

After completing the Lakebed Temple, Link and Midna will 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. This leaves Link trapped in his Wolf state.

Firstly, Link must travel to Hyrule Castle to meet with Princess Zelda who is the only person who can remove the curse on Midna. Zelda then sacrifices her physical form to ensure that Midna lives on. In return, Midna feels less selfishness towards helping Link save his world and agrees to help him find the Master Sword, which is the only means by which the curse can be lifted, as the sword counteracts the evil of the Shadow Crystal.

Deep within the Sacred Grove lies the Master Sword, which Link and Midna finally find after much trouble with Skull Kid. As Link approaches the sword he is transformed back into his human form; the curse of the Shadow Crystal is broken, and it coalesces in Midna's hand. She then explains that with this magic crystal he is able to transform into his wolf form at any point, as well as using Twilight Portals to teleport great distances. However, Midna warns Link that he must not transform within the view of bystanders.