A Curse of Nightshade by Amber Lynn Natusch

I made a deal with a devil once, and it cost me all but a shred of my soul.

Now, cornered in an alley by a sexy, silver-tongued demon who wants something other than my final tie to existence, I’m faced with a choice.
Since escaping the demonic realm six months ago, I’ve honed my skills and my rage hunting the craven evils of New York City alongside the witches who saved me. Vigilantes, they call us. Cold-hearted killers. Monsters of the night. If only they knew how true that was—what our pasts have shaped us to be.
Mine forged me into the most powerful Daughter of Fire ever, which is exactly what this hell-spawned charmer claims he needs to kill the demon who stole my soul. Together, Zen insists, we can share in revenge–but on one condition. I must bind myself to him, an unthinkable deed with deadly ramifications at best. He’s playing every beguiling card in his deck to lure me in, but I’m no fool. He may be the enemy of my enemy, but he’s still a demon.
And I won’t make that mistake again.