Lakesedge by Lyndall Clipstone

A lush, gothic fantasy from debut author Lyndall Clipstone about monsters and magic, set on the banks of a cursed lake, perfect for fans of Naomi Novik and Brigid Kemmerer. When Violeta Graceling and her younger brother Arien arrive at the haunted Lakesedge estate, they expect to find a monster. Leta knows the terrifying rumors about Rowan Sylvanan, who drowned his entire family when he was a boy. But neither the estate nor the monster are what they seem. As Leta falls for Rowan, she discovers he is bound to the Lord Under, the sinister death god lurking in the black waters of the lake. A creature to whom Leta is inexplicably drawn… Now, to save Rowan-and herself-Leta must confront the darkness in her past, including unraveling the mystery of her connection to the Lord Under.