Broken Veil by Jeff Wheeler

Rescued from a world of poverty, Cettie Pratt has avoided a bleak destiny—until now. Deceived and manipulated, she has been groomed for the ultimate betrayal: to destroy her best friend and stop peace from uniting two war-torn worlds. Her path leads her to a mysterious underworld where appearances can be deceiving.

Sera Fitzempress knows the value she has to her enemies. As heir to the empire, she must keep her foes at bay and prevent them from unleashing a being of unspeakable evil upon the world while fighting a brutal war. But her enemies are more cunning than Sera expects, and the key to their plans is none other than her best friend.

Neither woman knows what to believe. Neither one knows if she can trust the other. Both Cettie and Sera have made decisions that have irrevocably changed them. But the decisions they have yet to make will determine the fate of their world…