She's the Duke

As if reincarnating into a whole new world wasn't enough, Lewin was reborn to a father so bent on having a male heir that he used dark magic to disguise her as a boy. Over 20 years later, she finds an unexpected ally in a freshly-transmigrated fellow Korean girl. However, this new friend delivers shocking news: they're in a tragic BL novel in which a love triangle will ultimately lead to the apocalypse! Still, if Lewin can seduce the male lead first, they may just manage to save the world...

(Source: Tapas)