Mithral Ore is a ingredient found from one of two Ore Veins in Grymforge. It is used for crafting adamantine armour or weapons, which all have the Reeling effect. Instructions and recipes can be found in the book The Great Furnace of Grymforge and the crafting process is done at The Adamantine Forge.
The items that can be crafted are: Adamantine Mace, Adamantine Longsword, Adamantine Scimitar, Adamantine Shield, Adamantine Scale Mail, and Adamantine Splint Armour.
Crafting one item uses up one Mithral Ore and only one Ore can be obtained from each vein, meaning only two adamantine items can be crafted in any one game.