Saigo no Akuyaku ga Inma ni Yarereteshimatta

A reader finds herself inside a canceled adult novel as Levi, a frail extra born into a family of knights, a position that all but guarantees an early death. Her strategy is to stay invisible and never cross Prince Damian before the rebellion he is destined to lead. That plan unravels when a lust demon targets Damian, and although Levi manages to cleanse the demonic energy from him, she only does so after a night neither of them intended, leaving the throne-obsessed prince fixated on the one woman he should never have touched.