The Big Bad Duke by Sadie Bosque

Out of the darkness…
Leila was meant to be a healer, not a killer. Taken from her home at a young age, and having gone through hell, she became an evil secret society’s most dangerous weapon—silent, beautiful, deadly. Now she has one chance to earn her freedom: seduce the Duke of Wolverton… and end his life.

…and into the fire.
The Duke of Wolverton has spent fifteen years haunted by the ghosts of his past. But when he inherits the title and the mantle of Erebus—leader of the Shadows—he casts aside his solitude and takes up a single purpose: revenge. Every soul bearing the Brotherhood’s mark will bleed.

Until he meets her…
She was sent to kill him—but instead, she ensnares his soul. She’s not the monster they made her, and he’s more than the vengeance that drives him. But walking away from who they’ve become may be the deadliest choice of all.