"Myths abound about armored phantoms that terrorized brave heroes. This helmet looks 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 Coliseum Ruins. The buried treasure is located on the ground floor right along the wall at northwest portion of the ruins. Link can use Magnesis to easily spot the chest and pull it out of the ground. Be sure to check our Interactive Map for a more precise location.
The Phantom Helmet is part of the Phantom Set, which also includes the Phantom Armor and Phantom Greaves. When individually equipped, they will give Link an Attack Up boost. None of the items in the set are enhanceable or dyeable.