Returning back across the eastern swamp cave, avoiding the Green Dragon's den, head to Kol - we're gonna go ahead and turn in that Silver Harp since we really don't have that much use for it.

Starting at Kol, look to the southwest for a narrow pathway of trees cutting through the mountains. Follow that path west, then north until you reach a grassy clearing at the tip of a peninsula. You'll find this old guy waiting for the harp - he'll specifically ask for it if you come here without it. Claim the Staff of Rain and, together with the Stones of Sunlight that you should've found if you went back to Tintagel with a Magic Key and opened the path to the castle's basement, you'll have two of three pieces needed to open the way to the final area (which we DEFINITELY aren't ready to visit yet - we still have stuff to find elsewhere).