The Meeting of the Waters — Caiseal Mor

Brave, copper-haired Aoife was the daughter of a king, a bold young woman full of life and mischief. But on one winter's night she and her brothers took part in an act of careless mischief with consequences they could never have imagined -- and a deadly blood price must be paid.
In the forests to the west, a deadly force is stirring. Off the shores of Innisfail, a new enemy is fast approaching. The druid Dalan has been sent to unite two squabbling kings in the face of this overwhelming force, but chaos and confusion confront him at every turn. As dangerous bargains are made and broken, and truces struck and disregarded, Dalan begins to suspect that an even greater enemy is moving against Innisfail.
The last of the Watchers are growing bored. But mortals are an interesting game.