"Hydlaa".
As a sidenote, Vodúl isn't known to the Yliaki, though, just to the players. He's not even an option in the CC, whereas the BF, which isn't known in Yliakum, either, is.
Edit: also, the rock doesn't need to be resilient to the crystal's radiation: the part of the crystal that radiates can be expected to be lower than the ceiling (like an optical fiber, the radiation likely exits at the end only). Given that it takes only 12 steps to where the radiation ceases to be powerful enough to dissociate matter, the amount of this protrusion doesn't even have to be big. This also means that the crystal wouldn't be able to create a cave larger than with 12steps (possibly metres) radius.