Saigo no Akuyaku ga Inma ni Yarereteshimatta

When the reader wakes up as Levi, a sickly extra from a family of knights in a discontinued R-rated novel, she knows her days are numbered. To avoid being killed by the villainous Prince Damian during his future rebellion, Levi plans to avoid fighting him and live in the shadows. But fate takes a carnal turn when a lust demon attacks Damian. Although Levi successfully purifies the demonic energy inside him, it's only after their unplanned night of passion. Now, the ruthless prince who only desired the throne is dangerously obsessed with a woman he was never supposed to touch. (Source: Tappytoon) Note: First released in English, with the initial eligible release in Japanese followed by Korean, making it a manga by our standards.