Morrighan by Mary E. Pearson

Before borders were drawn, before treaties were signed,
before wars were waged anew,
before the great kingdoms of the Remnant were even born
and the world of old was only a hazy slate of memory told in story and legend,
a girl and her family fought to survive. And that girl’s name was Morrighan.

In Morrighan, a prequel novella to Mary E. Pearson's The Remnant Chronicles, a girl and a boy from enemy camps meet, fall in love--and set history in motion.