The Druid's Binding - M. Vixen

Ailith is in the middle of a heist when a mysterious figure breaks into the same vault and unleashes a deadly magical disease upon the world.
 
But the disease proves to be only the start of her problems. Haunting her every step is the devil Arkhael, who tries to tempt her into signing an infernal contract—her services as a thief in exchange for the cure.
 
Staying one step ahead of the devil and not giving in to his honeyed words becomes harder and harder as her time starts running out and the disease becomes more aggressive. Aware that not just her life but her very soul might be at stake, Ailith faces an impossible task: trying to beat the devil at his own game.