Black Unicorn — Tanith Lee

It was big and beautiful and so black that it was like a hole in space, and it was completely impossible. Unicorns didn't belong in this world except in legends. But there it stood, radiating magical power, in the shattered wreck of the party.

Nobody knew where it had come from, or what it wanted. Not even Jaive, the sorceress, could fathom the mystery of the fabled beast. But Tanaquil, Jaive's completely unmagical daughter, understood it at once. She knew why the unicorn was there: It had come for her. It needed her.

Yet she was the girl with no talent for magic. She could only fiddle with broken bits of machinery and make them work again. What could she do for a unicorn?