Plane of Isahnd
The Glass Veil is supposedly an artifact created by the mythical Isahnd that prevents the intrusion on the plane of Isahnd. The Veil prevents creatures from entering the plane, even mages that have learned to travel the planes are originally from Isahnd cannot return. Supposedly, even deities themselves are blocked by the Veil. While they can grant spells to their adherents on the plane, they cannot actually manifest on the plane.