Vahanatai would be more interesting, I'd think. They live in caves, and they have earth magic, but otherwise nothing in common with Agartha, really. They tend to be armed with wave blades and shuriken
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Alternatively, as the ages progress they should *acquire* the ability to recruit Vahnatai - who tend towards earth/astral/nature, or earth/astral/holy (two types of shapers). Human mages would do death/fire/water/air (since that's basically what "magic" does in Exile.)
Crystal souls are available as a national summon. They're immobile, but magically powerful incl a holy of 4.
Re: New Nation(s): Exile/Avernum
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