The Pale Lady

From the publisher:

Deep in the woods—any woods and every woods— lives a witch known as the Pale Lady or the Flower Mistress. She isn’t a witch, but a faerie queen—an evil faerie queen. As you can imagine, the finer points of that distinction are lost on the peasant farmers who lose their children to her malicious ways. Her domain is sealed away most of the year, but every spring the veil between worlds is thin enough that the Pale Lady can send out her army of emaciated animal thralls to steal the sons of honest people.