The Lost Mask. The Bone Mask Cycle, #2

The enormous corpse of the Sea Beast has breached the walls and an invasion from across the sea is threatening. Cut off from the mysterious golden egg and bereft of the hope of ever seeing his family again, Ain remains in prison, at the mercy of the new king who obviously wants something from him. Meanwhile, Flir and Luik struggle to contain the illness spreading from the Beast, as Notch and Sofia begin their desperate search for her father.
Charged with hunting down Vinezi, Flir and Luik find themselves thwarted at every turn while Notch and Sofia discover twisted magic is at work in the Bloodwood where the strange Sap-Born may prove to be the worst threat they have encountered yet.