Gwyddno Garanhir was the king of the Lost Lands, and creator of many beautiful and powerful objects (i.e., The Crystal Sword). He is locked away in his glass castle when Will Stanton and Bran Davies meet him in Silver on the Tree. He is slowly succumbing to madness and would have, if not for the dreams he says to have dreamt. He would not have given the sword to Bran if not for the dream he'd had about him beforehand.
Gwyddno is thought to have vanished with the rest of the Lost Lands.