Curse of the Forgotten City(Emblem Island‪)‬

The second book in the Emblem Island series from powerhouse author Alex Aster. In this tale Tor, Engle, and Melda must stop a band of cursed pirates from taking over their home.
Tor is adjusting to life with the power of the Night Witch, especially with his best friends Engle and Melda by his side. But when a mysterious girl washes up on shore claiming a band of cursed pirates is on their way to Emblem Island, life changes fast.
The girl, Gemma, is from an underwater city that was destroyed by the terrible Calavera pirates and she warns Tor they are now on their way to his home.
The trio of friends must come up with a plan to stop the pirates from getting an ancient relic that would give them the ability to control the high seas, while also protecting all they love from the new danger.