Shadowland: The Immortals 3 by Alyson Noel
Damen and Ever travel to Summerland in search of an antidote to reverse a powerful poison. But instead of the cure they seek, they find something far more sinister: the truth about their existence and the fate lying in wait for an immortal's soul. Now, with Damen fighting to save them from the Shadowland, Ever turns to magic, hoping to break Damen's curse. Along the way, Ever encounters the mysterious Jude, who shared a complicated past with her in a previous life. She always thought Damen was her soulmate and her one true love...but now their love is about to be tested like never before.