Webs of the Endangerment
Arpie needed a name for the enemy land on the borders of the Vale Perilous, so he used this one. Arpie likes folklore, but the only thing he could actually remember about Tyno Helig was that it had something to do with a rock.
Sir Percy begins the story as a prisoner there, but a well-kept one.
The Fierce Wall separates Tyno Helig from its enemies (The Empire), and the Vale in particular.