Dark Tide (Waterfire Saga, Bk. 3)

Six mermaids battle to save their hidden world.

Leaders of a resistance to the barbaric new regime, Serafina and Neela, are working hard to destabilise Vallerio, the new ruler of Cerulea. Becca, Ava and Ling are still hunting for their ancestors' talismans; the keys to keeping an ancient evil locked in its icy prison. Astrid finally returns home to Ondalina to find that all is not as she left it, and a sinister human with empty, black eyes offers her power she never dreamed of ...