Waterfire Saga: Dark Tide Hodder children's books

Serafina, once a lost and uncertain princess, is now a confident leader of the Black Fins, a ragtag group of soldiers trying to sabotage her traitorous uncle and his death riders. While Vallerio and his wife plot their takeover of all the mer realms, Sera works on enlisting allies for battle. Meanwhile, her friends Neela, Ling, Becca, Ava and Astrid face challenges of their own. Time is running out in their search for the four talismans that still must be found if they are to defeat the ancient evil that destroyed Atlantis.