The Forbidden Land: Book four of the Witches of Eileanan;

After her adventures with the League of the Healing Hand, Finn the Cat finds her life at Castle Rurach boring. Snowbound for the winter and faced with the prospect of being molded into “Lady Fionnghal” by her mother, she pines to be on the battlefront with her father against the sea demons.
 
But Finn’s talents are needed elsewhere. Summoned by the Righ, Lachlan the Winged, she must embark on a perilous journey into the Forbidden Land. Imprisoned in the Black Tower is a rebellious prophet whose beliefs have made him an enemy among his own people.
 
Now Finn must help rescue the one whose words can free a land enshrouded in darkness.