So Ethniu is the daughter of Balor and has the Eye of Balor.
That's interesting...
I guess it could just mean that she inherited his power, the way Harry inherited magic from his mother, but the Eye of Balor seems to be implied to be a singular, unique thing.
So is the Eye of Balor a magical object of great power, equivalent to the Blackstaff, Swords, Noose, etc.? The description of Ethniu's face sounded like her eye might have been replaced.
An artificial eye with magical powers... kind of reminiscent of the Gatekeeper...
If the Fomor are linked to the Outsiders in some way (their attack seems coordinated with the Outsiders' renewed/increased attack on the Gates) maybe the Eye is some kind of "anti-reality weapon"?
