Serafina prepares Ling, Neela, Becca and the rest of the Black Fins for war, to stop the Volneros from seizing the mer realms. But only when sea sisters Ava and Astrid find the remaining two talismans will the combined power of all six be strong enough to stop the devastating attack on the underworld. No charm will protect Ava as she faces the murderous Okwa Naholo spirits that lie deep within the swamp of the Southern Sea. Meanwhile, Astrid will have to go against the strongest of the Six Who Ruled, Orfeo, if she's going to take his flawless black pearl, the most powerful talisman of them all.