It's not said.

It could be old age, maybe she was already very old (multiple centuries) during her travel with Gulool Ja Ja 80 years ago. It could be sickness or being killed by some monsters (I can see her accepting the regulator but not wanting to use soul as safeguard against death).
At the very least, she seemed to know Sphene, and even if she was opposed to her goal, they didn't seem to have a "bad relationship", so I don't think she was killed or anything like that.