The Nightmare Queen by C.V. Betzold

Vellar’s heir to the throne and greatest disappointment is known for being stubborn and disobedient. Thrown into battle against a genocidal tyrant, Rorin and his out of practice army are forced to seek out help from the one person he knows only rumors of.
Obsidian’s queen has been elusive for twenty-five years. Just the way she likes it. A fan of striking fear in the hearts of men she’s shocked to find that there is a prince daring – or stupid – enough to seek out her aid. Now with him willing to plead his case at her feet in exchange for a high price – a decision must be made.

She’s his final bullet in a game of roulette he can’t stand to lose and it all banks on her willingness to be generous.