Somebody just dumped the whole plot on a forum I follow, so here’s what I recall. The core twist is that Cayena isn't just reborn into this villainess role—she’s being actively manipulated by the real mastermind, her brother Revellon, who orchestrated their mother's death and framed the original queen. He’s been pulling her strings since the beginning, which reframes every nice thing he does in the early chapters as pure calculation. Also, she doesn't end up with either of the main male leads initially presented. The narrative sets up the Crown Prince and the Knight Commander, but she forges an alliance with a third character, the supposedly 'weak' neighboring prince, to dismantle the entire system. The big emotional spoiler is that her 'memories' of her past life as a modern reader might actually be implanted suggestions from Revellon's mage, making her whole sense of foreknowledge another layer of control.
If you're reading for the romance, that's the biggest upset—the official pairing isn't who you think for about two-thirds of the story. The story shifts into a political thriller where Cayena has to untangle the real conspiracy without knowing which of her own thoughts are genuine. Knowing this ruined the 'transmigration revenge' vibe for me, because the central horror becomes psychological entrapment rather than just overcoming a bad destiny. I skimmed ahead on some aggregator sites and the art for the later arcs gets incredibly dark, with Cayena's marionette imagery becoming literal in some nightmare sequences.