Shadowbaby — Barbara Hambly

A novelette of 15,000 words; a sequel to Hambly's Dragonsbane (Winterlands) series. Everyone knows that when the faerie-folk of the marshes kidnap a human child, they replace it with a changeling, a shadow-baby. When John Aversin, Dragonsbane and Thane of the Winterlands, hears that the inhabitants of a local village are accusing a changeling child of murdering its human foster-mother, he is alarmed and appalled. He knows that changelings don't kill their foster parents - and that if the villagers harm the changeling, the shadowy faeries are capable of taking horrible revenge.