From Diablo Wiki
The Great Eye, possibly also known as the Sightless Eye, is the deity of the Sisterhood of the Sightless Eye.
Not much is known about this deity, if it's an actual being like the Amazon pantheon of gods, or if it's more a concept that the Sisters worship, or follows.
The Sister's military leader, Kashya has been known to call the Great Eye impenetrable - a slightly odd attribute to apply to an abstract idea, implying the Eye is actually a sentient being of some description.
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