"Myths abound about armored phantoms that terrorized brave heroes. These greaves look just like the ones they supposedly wore. If sold, this rare find can’t be replaced."
As part of the EX Treasure: Phantasma side quest, Link will find this piece of armor at the Hyrule Garrison Ruins. More precisely, they are located in a treasure chest that is buried underneath an inactive guardian at the southeast end of the ruins. Be sure to check our Interactive Map for an exact location.
The Phantom Greaves are part of the Phantom Set, which also includes the Phantom Helmet and Phantom Armor. None of the armor pieces are enhanceable or dyeable. Individually, when equipped they will each give Link an Attack Up bonus.