Elf Princess

Their world had been full of magic once. Cities flew in the skies, forests were alive and teeming with all kinds of phenomenal beings, and a spell could make fantasy become real. It all changed when the barrier between realms collapsed and the beings of the Abyss fell upon them. It was impossible to defeat those dark creatures without sacrifice, and the world changed for it. In a thousand years, magic waned, races died out, and monsters started roaming the land. Only two nations remained.
The elves of the Eternal Forest and the humans of the city-state of Haven, and for most of their existence these nations had been enemies. Now, if they wanted a chance to survive, something would have to change, and the leaders of these nations knew it. When a band of mutated monsters starts hunting the villagers bordering the Eternal Forest, King Julianus sees an opportunity to put one of his plans in motion.
He sends his second daughter, third in line for the throne, to the elves to get their help to put a stop to the creatures. More than that, he sends her as a bride for princess Eliana, hoping that union would cement the peace between their peoples and ensure they could work together. The one who won't be very pleased about it is Emma. She fled her father's court to escape an arranged marriage before, joined the army, and became a hero in time. Julianus knows his daughter though and understands her well enough to say she will see the historical opportunity they have before them.
It doesn't mean things will go as smoothly as he might want them to.