Reign of Darkness by Ariana Nash

“Know your place, Nikolas.”
“Under the heel of your boot, Your Highness?”

After a terrible inferno destroys the Caville palace, and with the people of Loreen turned against him, Prince Vasili is left with no choice but to flee his ancestral home, leaving his brother Amir to proclaim himself King.
But Vasili’s life isn’t the only one left in ashes. King Amir has not forgotten Nikolas Yazdan—a lord’s bastard son and Vasili’s favorite pet. With forbidden power in his veins, Amir has become more than Niko can fight alone.

Thrust together with Vasili once more, Niko must travel south, to a land of pirates and thieves, of mystery and magic. A land of opportunity for a manipulative prince and his reluctant assassin.
But just as Nikolas begins to believe Vasili might be worth fighting for—that Loreen can be saved with Vasili on the throne—the dark flame trapped in the royal bloodline unravels its ancient bonds.
One prince must control the unrelenting power desperate for its escape. The other must die.

Niko thought he knew which prince he could trust, but he’s been fooled before and as Vasili’s true motives are revealed, Niko finds himself at the mercy of Prince Vasili once more. Only this time, there’s more than love at stake. The fate of the world rests on Niko discovering the truth about the flame, about the cursed Cavilles, and his place in their doomed destiny.