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 arrived at Lanaryu's Spirit Spring, where they were confronted by Zant, who cursed them both when Midna refused 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 state.
Deep within the Sacred Grove lied the Master Sword, which Link and Midna finally founnd after much trouble with Skull Kid. As Link approached the sword he transformed back into his human form; the curse of the Shadow Crystal is broken, and it coalesces in Midna's hand.
By using the power of the Shadow Crystal, Link gained the ability to transform into wolf form at any point, as well as use Twilight Portals to teleport great distances.
Gallery
 Midna obtains the Shadow Crystal... |
 ...and explains how its power may be used |
