this is usable only if you have the Wardens Keep DLC quest and have completed it. Upon returning to the Keep after successfully completing it, and having asked Dryden to stay on, Mikhail Dryden has a stall to the right as you face the keep. In dialogue you mention the strange ore and Mikhail becomes excited and identifies it as star metal. He will then offer to manufacture either a one-handed or 2-handed sword from the material called "StarFang".
Longsword stats: http://dragonage.wikia.com/wiki/Starfang_%28Longsword%29
2 Handed Sword stats: http://dragonage.wikia.com/wiki/Starfang_%28Greatsword%29