After thinking about it, the black rock is egg-shaped, sucks out all your magical power, and is something all spirits seem to know about but not want to talk about.
What if someone found a way to kill spirits, put them into a death-like state, or convert them into an inorganic piece of matter? What if sucking out all a person’s magical power until death was the egg stone’s way of trying to get enough energy to revive itself?
It’s that or it’s just an egg for a “dark” evil spirit or the like. Wish I could find out more than just make baseless assumptions and theorycraft.