Allure by Lea Nolan

Worst. Summer. Ever.
Emma Guthrie races to learn the hoodoo magic needed to break The Beaumont Curse before her marked boyfriend Cooper's sixteenth birthday. But deep in the South Carolina Lowcountry, dark, mysterious forces encroach, conspiring to separate Emma and Cooper forever. When Cooper starts to change, turning cold and indifferent, Emma discovers that both his heart and body are marked for possession by competing but equally powerful adversaries.

Desperate to save him, Emma and her twin brother, Jack, risk their lives to uncover the source of the black magic that has allured Cooper and holds him in its grip. Faced with the horror of a soul-eating boohag, Emma and Jack must fight to resist its fiendish power to free Cooper long enough to join their strengths and face it together, before it destroys them all.