The Black Metal Armor is a secret armor with Item Power: 50. It is obtained by casting Portal in the square in front of Lord Drowsk's portrait in the Painting Gallery in Norrab Castle. Beware, you have to fight through a Bah-Lorn to get it!
PRO TIP: Stay more than 1 square away from the Bah-Lorn and blast it with Fireball until it dies. It will not chase you down unless you directly engage it. (You will probably need to bring food or potions to refill your Magic or walk away from the Bah-Lorn to use your ring to refill your Magic.)