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Paral is likely possessed. The changed voice, changed eye color, Paral's deliriousness, the shuddering, all these are symptoms of possession. Which is really, really, really, not a good thing.
A possessing spirit, according to the priests, has to be kept somewhere-it can't hold itself together, and it can't move all that well by itself. So there has either been a possessing spirit sitting around in some layer of the sewers for gods know how long, or someone actually brought a possessing spirit to Attalin.
Perhaps more to the point, possessing spirits damage the host, and you can't remember any way off the top of your head to get it out. It's been done before, though. Hazardous to the host, and even more hazardous for the paladin doing the removing, but it has been done...