- "This thing is the embodiment of the evil magic that Zant cast on you. Careful... If you touch it, you'll turn back into a beast! This thing is too dangerous... It's probably for the best if we just leave it here, huh? But on the other hand... if we kept it, you'd be able to transform into a beast anytime you wanted..."
- — Midna
The Shadow Crystal is an item from The Legend of Zelda: Twilight Princess. It is an evil object used by Zant. When Link comes into contact with the Shadow Crystal, he is instantly reverted to his wolf form. The Shadow Crystal Fog found in the Twilight Realm is also composed of Shadow Crystals.
At one point, Link is implanted with the Shadow Crystal, and must seek the Master Sword in order to break the curse. After doing so, Link retains the Shadow Crystal, and can use it in conjunction with the Master Sword to transform between his wolf and Hylian forms at will. The end of Link's Master Sword sheath has the Shadow Crystal design on it, so it may be where it is held when Link is in his Hylian form.
When Zant confronts Link and Midna at Lanayru Spring, he forces Midna to the ground and prepares the Shadow Crystal to impale her with; however, Link attacks the Twilight King and takes the hit instead, becoming trapped in his wolf form as it sinks into his forehead. Princess Zelda later explains to him that the Shadow Crystal is composed of evil magic and thus only the Master Sword, the blade of evil's bane, can remove its dark influence and return him to human form. After recovering the sword from its resting place in the Sacred Grove, the object is expelled from his body. Midna is initially reluctant to keep the crystal, stating that it is too dangerous; however, she reconsiders after realizing that by keeping it, Link could transform into a wolf at any given time by using it in conjunction with the sword. She ultimately decides that, since Zant was "so kind" as to give them such a gift, they should be "thankful" and use it to the best of their ability. From that point forward, Midna can be called upon to transform Link at any time, with the exception of when he is within view of other people, as it would frighten them. Midna would later reference this when taunting Zant about how it was thanks to his earlier curse on Link that they are now present at the Palace of Twilight.
Inside the Palace of Twilight, a dense matter known as Shadow Crystal Fog can be found. It acts similarly to Link's own crystal; however, he cannot return to human form while submerged in the darkness. Once he acquires the Sols and fuses their power into the Master Sword, he can use it to cut through the fog, temporarily vanquishing it. On both occasions that he is fought, Phantom Zant explodes and covers the floor with this dark fog.
The Shadow Crystal makes a cameo appearance in Linkle's Tale: Her True Self, where it is found in the Palace of Twilight by Linkle and Midna who become mesmerized by it. When Midna touches it, it causes her to transform into her Twili Midna form. After defeating Dark shadows of herself and Zant created by out of control Twilight magic, Midna make a bold decision to smash the crystal. By doing so, she could release enough energy to contain the dark magic, at the cost of returning to her cursed self.