Kenyon, S: House of Fire and Magic

From #1 New York Times and international bestselling author Sherrilyn Kenyon comes an all-new world unlike anything ever seen before...
TAKE BACK YOUR KINGDOM
Unicorns and Dragons never mix. For centuries, they've been bitter enemies and their kingdoms at war. But desperate times call for desperate measures. Tanis's beloved older brother and heir to their throne has been slain by a human who continues to flaunt her brother's death.
To restore her family's honor, she's willing to make a deal with a Unicorn shapeshifter so that she can infiltrate the human world and recapture what was so cruelly taken.
But Dash Coeur de Noir isn't just any Unicorn, as the High King he's the most hated and feared of his breed and he's on his own mission when the Dragon, Tanis, crosses his path- to reclaim his sister's horn that was brutally taken. In the wrong hands, that horn could destroy the fragile peace that has kept the Thirteen Kingdoms from destroying each other.
In a world built on magic, blood, and betrayal no one can trust anyone. With a bounty on their heads and assassins and enemies out to claim them both, they must somehow learn to trust each other or watch their kingdoms burn.