The Islands of the Blessed (Sea of Trolls Trilogy, Bk. 3)

In the final adventure of Nancy Farmer's acclaimed trilogy, a malevolent spirit of a vengeful mermaid is wreaking havoc on Jack's village and it's up to him, the old Bard and Thorgril to confront and vanquish the restless draugr. But the task will not be easy and the three find themselves travelling once more with Thorgil's northman brother and his crew into the most dangerous of waters. Their quest to right old wrongs leads them from a village plagued by a hogboon to the fin folk land of Notland and via every danger in-between. Can they escape the perils they face and return in time to help undead spirit to find peace?