Thinking back on the tidbit about Lia’s poor vision in LLE, I’ve realized that yes it was a funny way to explain her clumsiness, but it was also brilliant foreshadowing for her new role at the end of AHC.
A lot of rambling ahead, because I’m actually partially sighted/ on the blindness spectrum, so my ass has a very personal investment in the representation and exploration of blindness in this series.
The goddess’s oracle is dead and here she is to replace her as the polar opposite in who she serves and what she’s given up. Like her, she’s been bestowed with the position of a seer, but unlike the oracle, she hasn’t lost her physical vision, which will probably affect how she perceives any prophecies going forward.
Let’s talk about the blind seer trope. Their physical sight, or rather lack there of, offsets their cosmic sight.
Without physical vision, they can’t judge the world around them off of appearance. They can’t see what they want to. So their visions of the future have more clarity. They sacrifice their physical sight for knowledge.
Then there’s Themis, goddess of justice, who is depicted blindfolded. Justice is meant to be impartial, hence the lack of sight once again.
Meanwhile in LLE, Rae who should have unparalleled knowledge of the plot fails as an oracle because she sees what she wants. She wants to see the emperor as this grandiose figure, so she pins that image to the most powerful character she’s presented with upon arrival to Eyam. But wrong dude!
In AHC, she’s so desperate to talk to the Great Goddess that she convinces herself that the ghoul maid is her envoy. She’s looking for a way out through another deal, and so she sees nothing else around it.
How much of that is the cursed diamond’s fault is up for debate. Key destroying his half of the pair and proving it had no influence on him leads me to think that it’s not as powerful in its corruption as it seems. Maybe it had no effect on him since he’s a god. But for mortals, I imagine it doesn’t work on its own and feeds whatever pre-conceived notions the wearer has. Tunnel vision like Rae mentions. It just so happens that said vision lines up with what the Great God wants to see of the world, and her previous actions made her more susceptible to it. 
And before I circle back to Lia, let’s talk about him.
Sure, he has his eyes gouged out, but mf is still a god with followers who will perceive the world through his teachings and thus feed back into the vision he has of it. See the most extreme case, Marius’s father who becomes his vessel and ceases to exist as his own person. 
Now with Lia, we have the extra layer of irony surrounding the fact that she says she doesn’t want to be dependent on others’ power and favour, and yet she takes the diamond. The diamond that is famously a sign of royal favour and devotion, and who’s part of a set that came directly from a God‘s power. If she really will assume the role as oracle, her power will be much more heavily influenced by the Great God than the original oracle’s was by the Great Goddess.
Hell, the fact that the diamond fastens around the throat makes it a dark twisted version of the light that appears when a truth potion is drank. Any vision she might have in the future could be what she wants to see rather than the full truth. And it may very well b even more warped by what the god wishes was true, just as Rae’s perception was when she wore it.
Also, in contrast to the god, the goddess isn’t physically present in Eyam. I imagine that the oracle was also her way of looking into the world from as impartial a stand point as she could manage. Obviously, if she’s trying to send in people to change the story, then the oracle’s visions were ones she never wanted to come to fruition. And even as she tries to change the story by sending in people who already know of it, she can’t control what they do. Hence, the many many versions of the book.
This is all over the place, but I say all this to say, I can’t imagine clever Lia just giving herself up to the Great God like that. We still have a very limited view of her through Emer, so who’s to say that she won’t be aware of his manipulation? Will we get to watch her grapple with his vision of the world? How about her own? Would she be willing to give up her physical vision if it came down to it to help Rae?
And what drove the original oracle into that role in the first place? Was she also some level of visually impaired before hand? If she had more sight, what prompted her to give it up in order to assume that role? I’d kill for more of her backstory.
AND DON’T GET ME STARTED ON THE FACt THAT ALICE WEARS GLASSES TOO
