I think it's the gem. I don't think she'll die though, that would be sad. Maybe she reverts back or looses her powers and reverts into a tiny spider and follows him.
@Malte considering the overall setting with many killing and killed stuff, she's not the only one who did that. and so far she seem the first who isn't dumb to keep going or trying to backstab, like your normal villain.
Plus it's her house/mansion. Who knows if people or the transported 'players' wouldn't barge in to loot whatever she is a monster or not.