For my fantasy story?+BQs?

Sounds interesting a tad cliche but interesting. Do some more research. And write a few different starting paragraphs and just see where this goes. Use the epidemic as a reason for why your war started. The tension from the epidemic could cause a war. Also, I am a bit confused on the birthmark, instead maybe make all the royal heirs have raving black hair or violet eyes. Also if she is a peasant why would she marry the prince? like the other person said the first son is always the heir. Instead of the girl being a peasant she could be a little bit higher cl noble, but not high enough to be known. You could also make it that there is no war but k2 thinks k1 will attack so he sends the girl to get close to the prince and learn the army plans, but the king realizes who she is and wants to put her on the throne even though he knows that he will be shunned. You could even make q1 have an affair with k2 and not have k1 kill him, just the k1 always thought the prince was someone elses because he didn't even look close to him. Good luck! You could go a bunch of different ways with this. It is a fantastic idea and I hope you keep at it.