Tide Stealers — Kim Wilkins

After rescuing their baby sister, Una, from the witch princess Margritt, Asa and Rollo are once again, safe at their aunt Katla's home in Two Hills Keep. Asa is curious about the Moonstone Star and its magic, which she inherited when her mother, the Star Queen, was murdered by the evil court magician, Flood. But as Asa tries her hand at her first spell, a ferocious band of underwater pirates appears and steals the jewel away!

In order to get it back, Asa and Rollo must face malevolent ghosts, hungry sea giants, and a tide stealer who may hold secrets about their past, making them question what they thought they knew -- could Asa and Rollo's parents be alive?