Rise of Fire by Sophie Jordan

Luna and Fowler have escaped the kingdom of Relhok, but they haven’t escaped the darkness. When a battle against the dark dwellers mortally injures Fowler, Luna is faced with a choice: put their fate in the hands of mysterious strangers or risk losing Fowler forever. Desperate to keep the one bright part of her life alive, Luna accepts the help of soldiers from a nearby kingdom.

Lagonia’s castle offers reprieve from the dangerous outside world—until the king discovers both Fowler’s and Luna’s true ties to Relhok and their influence over the throne. Now pawns in each kingdom’s political game, Luna and Fowler are more determined than ever to escape and build the life they’ve been dreaming of. But their own pasts have a tight hold on their hearts and their destinies. Luna must embrace the darkness and fire within her before she loses not only Fowler, but the power she was destined to inherit.