Phantom Ganon's Sword is only usable by Phantom Ganon. In Ganon's Tower, there is a maze-like area where Link will have to fight Phantom Ganon multiple times. Whenever he defeats it in a room, the hilt of the sword will fall to the ground and point to the door that Link must use to keep going. Once Phantom Ganon has been defeated for good, Link will be able to pick up the sword and use it on a special wall, which will be destroyed and let Link continue.
An interesting easter egg is written on the sword itself. The words "Zubora Gabora" is written in Hylian, which is a reference to the blacksmiths in Majora's Mask.
