Shadow Crystal Fog is a substance from The Legend of Zelda: Twilight Princess. It is a black fog created by Zant, found specifically in the Palace of Twilight in the Twilight Realm. The Shadow Crystal Fog is, as its name implies, made from Shadow Crystals, which transform Link into his wolf form whenever he comes in contact with it. He can only be restored to his Hylian form when he is no longer surrounded by the fog.
Several times, Link is forced to walk through the fog in order to continue through the palace. The fog can be temporarily dispersed when exposed to either a Sol or the Master Sword when infused with the light of the Sols; there is no conventional way with which Link can permanently dispose of the fog. Spin Attacks and other ranged sword attacks performed with the latter are especially effective at banishing it. In cases of the fog acting as a wall similar to a waterfall, it is required that Link perform a Spin Attack to dispel it, if only temporarily. When Phantom Zant is defeated, he explodes into fog covering the center of the room in which he was battled.
Link's vision is severely impaired while immersed in Shadow Crystal Fog. Using the senses of Link's wolf form, however, will allow him to see marginally better. While using the senses, the fog appears with a semi-transparent light purple color.