คุณหนูสายลับไขปริศนา

Reborn as a fake heiress, a writer becomes a spy for a ruthless commander to solve a royal murder. While outsmarting her toxic adoptive sister and scum fiancé, she exposes a horrific truth: the "loving" duke is sacrificing his own children to demons for the throne! Trapped in a household of psychos, can this lethal fake daughter survive?