This is the complete trilogy of the Demon Queen Trials.
"I never thought I'd be singing happy birthday to myself in a dungeon. And yet when a sinfully sexy demon crashes happy hour, that's exactly what happens. He's known as the Lord of Chaos, and he's mistaken me for my succubus doppelgänger.
Happy birthday to me.
When he tastes my blood, he finally understands I'm mortal. And I realize we have something in common: we both crave revenge. So we make a deal: I can stay in the forbidden city to hunt for my mom's killer. In return, I'll help him get the vengeance he craves. I just have to pose as a sexy succubus. One problem--he's my number one suspect.
And that's increasingly hard to remember as he teaches me my role: seduction. With each heated touch I drift further into peril. And if I fall under his seductive spell, death awaits me."