Tanequil by Terry Brooks
The rightful High Druid of Shannara has been banished to the harsh world called the Forbidding by a treasonous fellow druid. Her only hope for rescue is her nephew Pen - but Pen is under seige as well. Both he and his parents are sought by the druids, who want to make sure that their magic will never help the real leader to return. Yet there is worse to come. For the dark magic that banished the High Druid into the Forbidding allowed the simultaneous transference of a fearsome denizen of the Forbidding back into the druids' world. And now there is a terrible peril in the Four Lands - a creature that can take on any shape, can kill at will, and is only the harbinger of a much greater, devastating invasion to come . . .