A hag eye is a magic item that appears to be a semiprecious stone. In fact, they are created by hag covens from disembodied eyeballs. Hag covens often give them to their minions to wear.
The hags of the coven are able to look through the hag eye as long as it is on the same plane as them. If the hag eye is destroyed, the coven suffer injury and temporary blindness.