Please help me with the name

My memories a bit fuzzy since I haven't seen it in months
The mc was Isekai into another world,but in here she was forced to pretend to be a man and dark magic was used to change her appearance ( she's a Duke and is best friends with the crown prince ). Then news hit that the princess went crazy and when the duke( mc ) met with her out of concern it turns out she was also Isekai into that world. However she tells the duke that they are in a dark romance bl,and to avoid the tragic ending the duke has to be the one to end up with the bl mc.
So the princess and she become bbfs basically and are training her( duke) to be the perfect yandere
Sorry for the long paragraphs