Caught in Crystal — Patricia C. Wrede

THERE STANDS A TWISTED TOWER...
hidden in the Windhome Mountains. Something is imprisoned there--the Sisterhood of Stars does not know precisely what, but something is bound tight with a wizard's spell so that it can never escape again.

Kayl is one of the few to have looked upon the Twisted Tower. She has no desire to see it again-she left the Sisterhood long ago, settling down to a quiet life. Her sword lies unused in a secret place beneath the stories of her hearth.

But something evil is leaking from the Tower. And now a sorceress and a wizard have appeared on Kayl's doorstep, demanding she take up the sword again.