Description
Mist clings to the surface of this shield and chills the arm of any who hold it. Nothing is known of who crafted the shield, or what strange sorceries cause it to remain chilly on even the warmest of days.
Just some runes in ancient Atlantean, spelling out the name.
Equipping this shield gives a boost to heat protection.
Source
The Mistmourn can be found in 
Legendary Chests.
Repair
This item can be repaired with a Legendary Weapon Repair Kit.