The Rise of Ashalla. Dead Souls, #12

As Edgar prepares to confront an ancient, rising power, he begins to fear that he has misunderstood Ashalla's true design... and that the cost may be nothing less than the world itself. If he is wrong, everyone he loves could be swept into the coming darkness. Meanwhile, Kate is being pulled relentlessly back toward her own time, even as Estella spirals further into madness. Two generations of Thaxos, separated by decades, are about to collide.
and the aftermath will alter the island forever. This is the twelfth book in the long-running Dead Souls saga. Each book ends with a cliffhanger, and readers are advised to start at the beginning. This is the 2022 edition of the original 2014 book.