The Thallain bother me for a lot of reasons. For one, creating purely evil characters tends to come with a lot of horrible implications and reifies a highly prejudiced, even fascist worldview. Sure, you can create characters that are essentially abstract metaphors or embodiments of things that are pure evil without running into these problems, but there's definitely an art to it, and the Thallain aren't doing it.
The games tell us that the Thallain came into our world after 9/11 happened and the War on Terror started. To a certain extent, I can see this. Post 9/11, people really did see the world in a different light, believing they were in danger of terrorists who were so deeply radicalized that they were functionally no longer human.
You can see this change in perception for yourself by comparing media before and after 9/11. You can see a definite shift in how antagonistic characters and peoples were portrayed. Moral complexity and proper personhood diminished; in its place, there was pure, unalterable malevolence that justified everything from torture to genocide.
But what the narrative of the Thallain overlooks is our own monstrosity. In accepting the narrative of inhumanly malevolent terrorists, we began to believe that inflicting all manner of cruelty and harm was not only justified, but necessary. The Thallain themselves are arguably objects to inflict that kind of monstrosity upon without any thought or reflection.
From a Doyleist perspective, the Thallain are very much part of the shift I mentioned. Before the Thallain were written into the game, there were no inherently bad or good faeries. All faeries could switch between Seelie and Unseelie courts, meaning that good fae could go bad and bad fae could become good. But for the Thallain, such change is impossible. Thus they reflect post-9/11 beliefs about terrorists who are capable of nothing but malice and evil.
I'm not completely sure how I would fix this narrative, but I have a few ideas. For one thing, I would have the post 9/11 atmosphere be a catalyst for many fae turning Unseelie, with many of them insisting that they're just doing what they must to keep Concordia safe. Being Unseelie would gradually become more and more socially acceptable as it became associated with personal strength and responsibility.
For another, the Thallain could have the potential to change for the better, or the claims that they're fully, completely evil could be fearmongering and propaganda. Or perhaps Thallain are ordinary fae who have given in completely to their Unseelie side that they've utterly lost touch with their Seelie side.
Oh, and one other thing that needs fixed? The way they're typically "uglier" than the Kithain. I think there are ways you could make them look different without it being a real problem, but the Thallain (and much of CTD in general, tbqh) has a major problem with leaning into disfiguremisia, racism, and occasional anti-environmentalism with evil characters. (Yes, using "ugly" animal features as a visual shorthand for evil is anti-environmentalist, because it demonizes the animals they come from.)
