It can be gained as loot from Baresh when the party returns to Ulgoth's Beard after successfully finding Balduran's Log Book. It offers a +3 armor class bonus and is 31 lbs lighter than normal Chain Mail.
|“|| Chain Mail +3
This perfectly crafted chain mail is the standard armor of deep dwarf elite Fighters. It is comprised of a mithral alloy and offers protection from all but the most determined attackers.