Looking for a shoujo

I can’t for the life of me remember the title or cover page. But here’s the plot:

The royal family all is gifted with innate magical abilities. Fl is red haired and the daughter of the previous empress but when her ability is found to be poison (all her bodily fluid are poison) she kills her mom by accident and then the new empress and her children like totally abuse her. Yada yada there’s a war and the knight from the other country comes to settle everything w a marriage. So the family decides to send the fl so assassinate him w her ability but on their first night it’s found that her poison just makes him healthier (he’s sickly so he always wears his armor). But yeah she’s very cold toward him and he’s kinda smitten (not outwardly but it may be in part to her ability.)