Firelight (Amulet #7) by Kazu Kibuishi

Emily, Trellis, and Vigo visit Algos Island, where they can access and enter lost memories. They're hoping to uncover the events of Trellis's mysterious childhood -- knowledge they can use against the Elf King. What they discover is a dark secret that changes everything. Meanwhile, the Voice of Emily's Amulet is getting stronger, and threatens to overtake her completely.