Kathleen, a half-fae, half-human girl, witnesses her mother's murder at the hands of King Theron of Romiaus, who enforces strict segregation between humans and the magical beings of Eithoria. Banished from Romiaus at age seven, Kathleen dedicates her life to revenge. She finds refuge with the Shadow Syndicate, a group of vigilantes, and becomes their top assassin, "The Shadow Assassin," mastering archery and bladed weapons under Nightgale's tutelage. As Kathleen navigates dangerous missions for the Shadow Syndicate, she finds herself caught in a love triangle between Vincent and King Viktorus of Eithoria. While she pursues justice for her mother and grapples with her own complex emotions, Kathleen must also contend with the political machinations and dangerous power dynamics of the two kingdoms. Her journey involves navigating her revenge, uncovering secrets about her mother's death, and ultimately finding a path toward self-discovery and reconciliation amidst the turmoil.