Water Series by Kara Dalkey

Nia, a young mermyd of the Bluefin clan, has had one wish all her life - to be Avatar in her beloved home of Atlantis. To be one of the ten Avatars that rule the undersea city is an honour and a great responsibility. Now at 16, Nia has a chance to see her dream come true ...Ages 13+.

After evil King Vortigern's men steal the seashell Corwin found, he meets Nia, a mermyd from the lost city of Atlantis, who is also seeking to reclaim the shell, for it holds the key to the survival of them both.

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... moreNia 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. less