Shadow Crystal

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After completing the Lakebed Temple, Link and Midna arrive at Lanayru's Spirit Spring, where they are confronted by Zant. Zant transforms Link into his Wolf form and takes the Fused Shadows from Midna. Zant summons the Shadow Crystal to display his power. In response, Wolf Link attacks Zant, only to be repelled and 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. This breaks the curse of the Shadow Crystal, and it coalesces in Midna's hand.

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

The Shadow Crystal makes an appearance in the Scenario "Her True Self". It was used to transform Midna from her cursed imp form into Twili Midna before helping Linkle save Zelda in the Palace of Twilight. After the battle was won, the darkness of the Twilight continued to spread and to prevent it from spreading further Midna smashed the Crystal to release enough energy to contain the darkness, causing her to return to her cursed imp form.

Trivia

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