Looking for a short story I read around late-90s. I'm pretty sure it was part of a short story collection in a book, and was not new at the time.
There's a witch or some kind of otherwise magical woman who lives out in a cave. There's also a man who comes to find her - I think it's implied that they used to know each other or maybe once had a romantic relationship before she became a witch. His country is at war with a particular group of people who have been pillaging his towns, and he's seeking her help to defeat them.
At the end, he ends up going into a special room in the cave to do the magic to defeat the opposing group of people. He has visions of all of them being sucked into the Earth and realizes that he is not only killing those specific individuals who attacked his country, but all people who belong to that group, including children and those who never participated in the war. Eventually the entire group is sucked into the Earth and they are wiped out of existence.
The price he has to pay for this magic is that the man goes blind. He finds this out when he wakes up after doing this magic, and the witch holds a candle in front of him which he cannot see.
Details are a little fuzzy because I was about 5 years old when I read this (but despite that, it was definitely not a story for children, it was targeted to an adult audience and not little kids like me scrounging around for old books in the basement). I specifically remember that the group of people he defeated were sucked into the ground, and that he goes blind as the price for working this magic.