What was that movie I watched in 1990??!!!

I’ve been trying to find/ figure out what movie I watched when I was 4 or 5 years old. It left a huge and lasting impression on me, and I’ve been trying to figure out what it was ever since. Nobody seems to remember anything like it, so I’m making this list of films that sound even remotely similar to what I remember, so that maybe, just maybe, a 26 year mystery might be solved.

If anyone out there reads this, and remembers a film with an older/ adult person telling two kids about a woman that lives in an underground kingdom, falls in love with a human, goes out into the sun and turns to salt and starts crumbling,...