If I remember right all nobles have black hair, and a daughter to a noble or a king was to be sacrificed. The father saved his daughter by hiding her away with a farm couple. She ends up in a desert with short magical people.
There is also an invading army conquers all lands. All soldiers can speak the tongue of the native people in land they are currently conquering. If I remember right if a country can hold out for up for 10 years they'd be integrated into the Conquering Empire rather than conquered. I read this book about 10-15 years ago, the book could be much older. The book was written in English but the languages of the book were fictional. Also if I remember right the invading armies uniforms were similar to that of roman armor. I could be wrong though. this may have been a teen/young adult book maybe
I remember one of the first battles all the black haired nobles were on there Their best horses (even if the horse had white socks, that was bad luck in their culture) if I remember right the nobles were so confident in their Cavalry. That all the Nobles families watch the nearby hilltops with all their Treasures. They lost the battle and Nobles were captured.
Set in Medieval Times. Book series.