To Kill a Kingdom by Alexandra Christo

Princess Lira is siren royalty and the most lethal of them all. With the hearts of seventeen princes in her collection, she is revered across the sea. Until a twist of fate forces her to kill one of her own. To punish her daughter, the Sea Queen transforms Lira into the one thing they loathe most―a human. Robbed of her song, Lira has until the winter solstice to deliver Prince Elian’s heart to the Sea Queen and or remain a human forever.