Article: After gaining the Master Sword, Link is cleansed of the magic that kept him in wolf form, obtaining the Shadow Crystal. Now able to use it to switch between both forms at will, Link is led by Midna to the Mirror of Twilight located deep within the Gerudo Desert, the only known gateway between the Twilight Realm and Hyrule. However, they discover that the mirror is broken. The Sages there explain that Zant tried to destroy it, but he was only able to shatter it into fragments; only the true ruler of the Twili can completely destroy the Mirror of Twilight. They also reveal that they used it a century ago to banish Ganondorf, the Gerudo leader who attempted to steal the Triforce, to the Twilight Realm when executing him failed. Assisted by an underground resistance group they meet in Castle Town, Link and Midna set out to retrieve the missing shards of the Mirror, defeating those they infected. Once the portal has been restored, Midna is revealed to be the true ruler of the Twilight Realm, usurped by Zant when he cursed her into her current form. Confronting Zant, Link and Midna learn that Zant's coup was made possible when he forged a pact with Ganondorf, who asked for Zant's assistance in conquering Hyrule. After Link defeats Zant, Midna recovers the Fused Shadows, but destroys Zant after learning that only Ganondorf's death can release her from her curse. Returning to Hyrule, Link and Midna find Ganondorf in Hyrule Castle, with a lifeless Zelda suspended above his head. Ganondorf fights Link by possessing Zelda's body and eventually by transforming into a beast, but Link defeats him and Midna is able to resurrect Zelda.

Now answer this question: What does Link obtain that gives him control over his transformations?
Shadow Crystal