Ever wanted to call someone an ugly old woman, but hag and crone didn't seem to fit? It's more common than you'd think. Mercedes Lackey's Arrow's Flight has the solution: beldame. Kris, who is very handsome, is avoiding many of the young women who want to dance with him:
"I'm dancing with you to keep from being devoured by them," he nodded toward a group of Court beauties languishing in his direction. "I can't dance just with beldames, Elspeth has to take other partners, and the only Heralds I can trust not to try to carry me off are Keren, Sheri, and you. And those other two don't dance."
Wiktionary defines beldame as a dated word for "an old woman, particularly an ugly one," though in this case I think Kris does not intend to call the women ugly.
Sometimes I think that fantasy novels use archaic or otherwise uncommon language just to remind you that, indeed, this is a fantasy book you're reading. Valdemar is located in another world, not a past time--why should they be using dated English words? It's just a convention of the genre, I guess.
In modern American English, beldame occurs once in about 110 million words, though in science fiction and fantasy it's much more common, appearing about once in seven million words.