The Stiehl assassin / Terry Brooks.

"The Skaar have arrived in the Four Lands, determined to stop at nothing less than all-out conquest. They badly need a new home, but peaceful coexistence is not a concept they have ever understood. An advance force under the command of the mercurial princess Ajin has already established a foothold, but now the full Skaar army is on the march--and woe betide any who stands in its way. But perhaps the Skaar victory is not quite as much of a foregone conclusion as they all assume. The Druid Drisker Arc has freed both himself and Paranor from their involuntary exile. Drisker's student, Tarsha Kaynin, has been reunited with Dar, chief defender of what is left of the Druid order, and is learning to control her powerful Wishsong magic. If they can only survive Tarsha's brother and the Druid who betrayed Drisker Arc, they might stand a chance of defeating the Skaar. But that is a very big if... as Tarsha's brother now carries the Stiehl--one of the most powerful weapons in all the Four Lands, and is hell bent on taking his revenge on everyone he feels has wronged him."-- Provided by publisher.