Tenderly, I Am Devoured by Lyndall Clipstone

Expelled from her prestigious boarding school following a violent incident, eighteen-year-old Lacrimosa Arriscane returns home in disgrace to discover her family on the point of financial ruin. Desperate to save them, she accepts a marriage of convenience… to Therion, the chthonic god worshipped by Lark’s isolated coastal hometown.
But when her betrothal goes horribly wrong, Lark begins to vanish from the mortal realm. Her only hope is to seek help from Alastair Felimath: the brilliant, arrogant boy who was her first heartbreak, and his alluring older sister, Camille. As the trio delve into the folklore of gods, Lark falls under the spell of both Felimath siblings.

Ensnared by a fervent romance, they perform a bacchanalia with hopes the hedonistic ritual will repair the connection between Lark and her bridegroom. Instead, they draw the ire of something much darker, which seeks to destroy Therion—and Lark as well.