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...which could be as simple as a reliable source of very bright light, protected somehow from Uz darkness-magic. Bright light is going to be somewhat demoralizing for all of them (especially considering the symbolism of surface light invading the caves, a la Yelm Babykiller's invasion of Wonderhome), and is debilitating to the trollkin.
Oh, and if you want yet another conspiracy theory: Repeated, successful invasions of the caves are going to be especially humiliating for the Karrg cult. There are lots of warrior cults among the trolls, but Karrg's Sons are especially dedicated to *protecting the home*. And they're failing at that, over and over. Who might be pleased to see them humiliated? Who might even have had a hand in bringing this about? Three guesses. (Hint: Z---- Z----.)
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