It is a new piece of headgear armor
A helmet much like the one Midna wore when she fought alongside the Hero of Twilight. If sold, this rare find can't be replaced.
- Armor Level 7
- Guardian Resist up 1
- This cannot be upgraded.
"The princess of twilight, whose stories are handed down alongside those of the Hero of Twilight... Her helmet can be found at the temple ruins soaked in the waters of Regencia River."
Midna's Helmet Location
The chest is in the Sage Temple ruins next to the Giant's Forest, The chest is on a small chunk of land submerged in the ground/water on the northwestern area close to the river proper. You need to use Magnesis to pull the treasure chest out of the ground.
See screenshot and map below for help to find Midna's Helmet.