Transformation (Water Trilogy, Book 3)

Nia and Corwin have each fought their own battles. Now they are Joined together to face their most dangerous enemy yet: Ma'el, the evil Avatar. In order to defeat Ma'el, they must retrieve a magical sword from Atlantis. But after a treacherous journey through the sea, they are shocked to discover that the sword is nowhere to be found.

As Nia and Corwin search for the sword, they realize that the good people of Atlantis are now enslaved by Ma'el. Worse still, Ma'el plans the same for Corwin's native land of Wales. Nia and Corwin must find the sword and stop Ma'el -- before it's too late.