A war has been waging between demons and elves for many years now and finally it has come to an end. The final assault lead by the demon prince (your character) was successful in crushing the elven defenses which allowed the demons to take control of the kingdom. The elves were forced to become prisoners within their own walls and homes, it was either that or try resisting only to die or worse. The demon prince wishes to make the elf princess (your character) his bride so that the elves will follow his command and rule. She is of course forced into this marriage and heavily coerced by the threats of her race being abused, tortured and slain if she does not cooperate.
So the relationship between the two of them would be very rough at first and non-consensual. She hates the demon prince because of what he has done to her race and kingdom, and she hates him for forcing her into this predicament. She can't turn her back on her people and thus is coerced into being his bride. The demon would use her to gain control of the elf kingdom - maybe even display how he has completely conquered their race by having the princess sexually service him as he sits in the thrown and deals with matters of the kingdom as the new king - demons approaching him and also escorting important/high ranking elves to him to talk about some things that need to be handled and the whole while as he talks and has these meetings the princess is forced to stroke/suck him off or he even pulls her into his lap to ride his demon dick.
It's not often that I'm into non-con, or rather I don't like to dwell on it. So I would like it to develop from there. She starts to enjoy the lewd acts he performs and how much pleasure he can bring to her body, even though she doesn't want to admit it her body will not lie. Slowly but surely she will start to absolutely love it and maybe even become addicted to his rough ways. I would even like a romance to form at some point, all that raw passion and roughness between them turning into something more.
The role play is heavy with sexual elements, but there will be a lot of story and plot elements to consider as well. It has to do with a war and running a kingdom after all, and since he is the new king and she is his queen then politics will come into play.