Dark Fog

Dark Fog is an element that is found only in the Palace of Twilight, the eighth dungeon in Twilight Princess. This mysterious haze-like element is comprised of Shadow Crystal created by Zant's wicked magic. It is scattered across various parts of the Palace, and if Link is to come into contact with the mysterious element, he will transform into his wolf form, and will be unable to have Midna transform him back into his human shape until he leaves the fog or clears it. Once Link obtains the Master Sword, he will be able to clear away Dark Fog using the Spin Attack. Midna mentions that the Dark Fog is what caused Link to become a wolf in the beginning of the game.