As you start the 'Forged of Star-Stuff' quest, one of 's many side missions, you need to find a Starmetal smithy. Although the side quest is triggered as soon as you pick up the first piece you find in the Living Lands, you're given no direction whatsoever.
There is only one place in the game: a blacksmith capable of working with such a metal. However, depending on where you are in the campaign, you might need to put a good amount of time into getting there.
When you do, you will be able to forge two powerful unique weapons. They're not among the best weapons in altogether, but they will still be worth the effort to add them to your collection.
To forge Starmetal in, you need to go to Forgemaster Dela in Solace Keep. This is within Gallawain's Tusks, the fourth and final region of the game.
As you progress through most of the campaign, you will eventually gain access to the region automatically. It will take around 15-20 hours to reach Gallawain's Tusks in .