Trials of Terraina Series by Vivienne Savage, Dominique Kristine

Demon summoner. Necromancer. Heir. As the Shade Knight of his noble house, Etienne is all of these things but also intensely loyal to those who need him. No challenge is too great for his powerful black magic, until an assassin begins picking off his estranged family one by one. Becoming the first female knight in all of Hindera is Raennia's greatest accomplishment... moreDemon summoner. Necromancer. Heir. As the Shade Knight of his noble house, Etienne is all of these things but also intensely loyal to those who need him. No challenge is too great for his powerful black magic, until an assassin begins picking off his estranged family one by one. Becoming the first female knight in all of Hindera is Raennia's greatest accomplishment. Those trials seem easy compared to her latest assignment: escorting a headstrong prince back to his kingdom and keeping him alive. He follows the goddess of death, she cherishes the goddess of life--his brooding at odds with her eternal optimism. Though they couldn't be more different, working together is the only way to unite their kingdoms, or else an evil force from the north will tear both nations asunder. less