I am trying to recall a fantasy novel that I did not finish. In fact, I think I only read the prologue. It piqued my interest, but I never got to the rest of the book. It was probably in the late 1990s that I started it (in English).
A young-ish woman is infiltrating the fortress of the evil lord, meaning to get revenge for some past atrocities. She stumbles upon a brother-sister pair of (red-headed?) mages and observes them from the shadows. Eventually, the mages decide they need to perform some kind of magical rite for their master. The woman whips off her dress and begins the ritual.
Seeing the witch naked, with sagging breasts and other imperfections, makes her seem a lot more human to the hidden observer. Up to that point, she had only see the magic users as mere evil minions.
What impact this had on her behavior, I do not remember. However, I think she got caught, and that was where the prologue ended.